Rarity sighed staring blankly at the paper, trying to think of a new idea for the dress. A few months ago the fashionetta experienced a surge of sales on her dresses and a brief bump in fame, but now that was starting to dwindle away. She wasn't quite sure why that was, as far as she or her friends (assuming they weren't just being polite... well she was reasonably sure Fluttershy wasn't)  could tell her dresses were as good as they ever were. The only explanation Rarity could think of was that her dresses were out of fashion? The thought was simply ghastly, but Rarity was never one to give up easily. If her ideas we're considered old hat, she would just have to come up with something the fashion world had never seen before and could never forget!
The only problem was that coming up with such an idea was a lot harder than deciding to come up with an idea. Rarity mulled it over, wondering how she could stand out. As the dress maker reflexively levitated some fabric over she was struck by inspiration. A way to stand out, and a way to not be soon forgotten.


Twilight sighed. "I don't know how you roped me into this."
"Because you are such a good friend you couldn't resit helping me and I of course, needed the best possible mage?" Rarity flattered "Besides, its not like I'm doing this for fun. Sales off my dresses have been dropping steadily and I fear the fashion world is starting to view me as old hat. I'll have to hit them with something they've never seen before and will not soon forget!" Rarity ended her speech by dramatically thrusting her hand out in determination.
Twilight sighed again, lowering her head a little "I have a bad feeling about this.. but I'll help."
"Thank you, thank you, thank you!"  Rarity said gripping Twilight's hands "Now lets hit the books!"
"I thought that was normally my line....."

The libary had been been reorganised recently so it didn't take Twilight to find what she was looking for. Spike was out fetch something for Twilight.
"Here we are." Twilight lifted the book towards, a purple aura surrounding it "The Definite book on Body Modification Magic: For the Unicorn who wants to look their best and other uses." 
"Thats a rather long title and slightly awkward title."
"Yes, yes it is." The book drifted into Twilight's hand. She noticed it was covered in a thick layer of dust, it wasn't a book Twilight had really read.  SHe examined the inside closely "Hmmm its a very old book and this kind of magic seems to have fallen out of common practice... I wonder why that is?"
Rarity snatch the book out of Twilight's hands "Oh I'm sure its nothing.... thanks again for finding it

Rarity read over the book. It was rather complex, and some of the most difficult magic she had seen.... It was daunting, but the fashion designer wasn't going to give up! And if need be she could always ask Twilight for help....

After Several days of reading and practice, She had a frustrating beginning but slowly it started to make sense.. some of it. She had also managed to make a few minor changes, though had reverted the. Now the fashionetta was going to see what she could really do! 
She stared at herself in the mirror. Rarity was wearing her usual short white dress that she usually had on when she went to see her friends. Rarity figured it would be good for showing off any changes she managed.
"Ok Rarity, you've been studying hard, now its time to put it into practice!"
Rarity concentrated as hard as she could and waved her hands. It was indeed more complex magic than she was used to performing, but Rarity could be pretty focused when she wanted to be. She felt a discharge of energy and opened her eyes expectantly.
A brief blue glow surrounded her for just a few seconds, then faded suddenly. Rarity waited. Nothing seemed to happen and Rarity sighed... but her dress felt tighter. Rarity looked down at herself and realised her breasts were expanding, and not only that, but getting rounder. Rarity smiled at her success, and examined the rest of herself. Look behind her, her hair was lengthening and her legs were stretching.  Rarity was giddy with success but using such complex magic she realised it was important not to lose control of it, and from the way her chest was pushing against her clothes, she realised she might lose a nice dress if this went wrong. Rarity focused again, this time keeping her eyes open. She pictured exactly what she wanted to look like, and exactly how this would go, and her body soon complied. Her hair was growing wildly, but now grew in an orderly fashion, in several rows straight down her back (while also looking like she had just washed and styled it), her breasts continued pushing against her dress but in a more controlled fashion, merely bumping against it, drawing attention to themselves without being too conspicuous, while also forming a small cleavage over the front of it (Rarity hadn't designed the dress to do that, but figured it was a good look), her legs grew longer, but were fashioned into long model legs rather than ending up like stilts. In the process of casting this spell Rarity had also gained several inches of height on her upper body, but figured it worked. She examined herself, and the look she had carefully sculpted. 

Rarity smirked, proud at her work and her figure, though she felt she might need to see Twilight to get more out of it. And this would be an excellent time for a field test.

Rarity walked slowly to Twilight's treehouse, savouring the looks she received. People were indeed staring, of both genders to boot. Rarity smiled, pretending to be disinterested, but slowly running her hand through her newly thick hair. Rarity was surprised how thick her hair had become, but was happy enough with it.

(This is going wonderfully!) Rarity thought (But I can't wait to see how Spike will react....). 

Rarity eventually grew bored of the oggling public and made for the library at a more respectable pace, those long legs already proving useful. In no time at all she was there. Rarity knocked. 
"Twilight I'm here! Ohhhh Spike! Could you let me in?" Rarity asked, a note of seduction entering her voice.

As she hoped, Rarity heard the sound of Spike running towards the door.

"Oh hey Rarity Twilight is-" Spike cut himself off, staring at the new Rarity in amazement. Spike was rendered completely speechless, unable to do more than stare up dumbfounded at her.

Rarity smiled,  placing her hand beneath her breasts, propping them up "Oh Spike, do you like my new look?"

Rarity could see Spike was staring up at where her face used to be.. which was noticeably lower than where it was now.

"Y-your beautiful..." Spike stammered out then his voice speeded up "Erm I mean you were beautiful before but now your... even more beautiful!"

"Why thank you Spike!" Rarity's smile widened. She let Spike drink in her appearance for a few moments, before Twilight came along behind him.

"Oh Rarity how did it..." Twilight asked before she could see Rarity in the door frame, before looking pretty surprised herself "... I guess it went well?"

"Yes, you could definitely say that" Rarity said confidently. Spike was rooted to the spot.

"Come along now..." Twilight said dragging him away, then turned to face Rarity. Spike still continued staring from the doorway, though Twilight was oblivious to it and Rarity was flattered by the attention

"So did you come here to show me your success?"

"Well actually I came for some help going further." Rarity said. Twilight looked surprised.

"Isn't this enough?"

"Welll... I have plans." Rarity smiled "And I need to figure out how to get the dress to adjust to the changes. If you look closer, you can see I did some damage to this dress, though I've done a good job covering it up. If I went much further I might have ended up shredding this dress!" She said, with a dramatic note at the end.

"And theirs more I want to do, but I think I might also need your help for it. I've been studying this book for days and I still feel like I've got a lot to learn."

"What could you possibly have left? You seem to be making very good progress on your own." Twilight said.

"Well I fear I've hit a bit of a wall. And I want to experiment with other expansion possiblites for a variety of dresses. Also I figure operating on a <i>grander</i> scale would make for some great advertisment and fashion possiblities." Rarity grew increasingly exicited as she talked, though Twilight still wasn't displaying much emotion beyond surprise.  

"Don't worry, I don't expect you to do all this for free, I'll gladly give you a share of the resulting profits. You could always use more books right?"

"Well I guess... I don't really see why this is nesscary, but I'll definitely see what I can do" 

"Thank you, thank you!" Rarity said, hugging Twilight, to the later's awakrdness. 

Like Rarity, Twilight spent the next few days reading over the book. It was somewhat easier for her and she got through it notably quicker than Rarity, but she had to admit it was difficult magic, and she still wasn't sure of any possible side effects.

{Ok, hopefully it won't be too hard to at least match what Rarity did.)

Twilight concentrated and waved her hands. It was tricky magic, but she had woven more complex spells than this, sometimes unintentionally. A brief purple glow surronded her, then faded. Twilight waited, nothing happened.

(Hmmm... I thought I had performed that correctly...)

Twilight read over the step again. She wasn't aware she had done anything wrong, but performed the same steps again. As soon as she finished the purple glow reappared, this time brighter than before, notably brighter than when Twilight normally casted a spell.

(I hope this is normal...)

Twilight immediately felt her dress tighten. Her breasts almost immediately went from fairly unotable to pushing up against her clothes and she could feel their shape changing, getting rounder, firmer and heavier. Twilight felt rather embarrassed and as the top of her dress ripped.  Twilight hastily cast the spell she had prepared for her dress, so that it would be able to keep up with the rest of her, though it didn't seem to be able to catch up with her escalating chest.  Flustered, Twilight attempted to cover them with one lengthening arm and look behind her.
She could see that her hair was rapidly lengthening and thickening, pouring down her back like a rushing river. Twilight initially didn't mind, but it was multiplying in length and thickness and would soon hit the floor. She was about to try and fix this, when she her own legs creaking and looked down, alarmed to her her legs rapidly rising up. The changes individually would be little more than bothersome, but their speed, intestity and the fact they were panicked Twilight. She felt out of control and let out a distressed yell, as the weight in her chest increased and she was racing towards the ceiling. Twilight ducked down, both aiming to avoid knocking her head, and trying to manage the increasing heaviness in front of her. It was starting to draw her to the floor, but Twilight focused, applying Telekinesis to lighten the burden she felt. With this pressure off her, the mage sat down, her height still nearing the ceiling. Seeing her legs in front of her,  she realised that proportionally they had to be  at least twice their normal length probably more, and were pushing again her wall. Concentrating she willed their growth to stop and after a few seconds they. Breathing a sigh of relief, Twilight looked behind her, only to find her hair had become thicker than her entire body, and was so long it stretched across the room in several directions, and still increasing in length by the second. Looking at her door she could see it was rolling onto the landing.
"Oh no you don't!" Twilight said aloud in determination, her voice having notably greater volume.
Twilight tried to rein her hair in with telekinesis, but as she pulled it in, its length kept increasing.  Sighing she simply concentrated on ending the effect and after a few more agonising seconds it did so. The librarian sighed.
"Well that was awkward" She complained bitterly out loud. "Even my voice is different."
Twilight sighed. She tried to dispel the magic on her, but to no effect. She gave an angry growl, frustrated that she was trapped in her own room. If she moved much at all she would break it apart.
"This reminds me of a story I read... and yet it feels like I've done this before....."
Suddenly Spike came running up.
"Twilight I heard strange sounds coming from your room are you-"
"CLOSE THE DOOR!" Twilight yelled, covering her exposed breasts.
"Oh I thought you... hey what are you doing?" Spike said, confused. In spite of himself, he couldn't help gawking a little at the sight of the new Twilight.
"Spike! This is not a good time!" She telekentically pushed him out of the room and closed the door.
Spike stared blankly, even more confused by Twilight's uncharacteristic rudeness. Twilight herself seemed aware of it.
"I'm sorry Spike! I was doing something for Rarity, I'll explain later!" Twilight yelled apologetically.
She felt a little guilty brushing her assistant off like this, but she didn't want to make an embarrassing situation even more awkward.
(Ok I have to figure out a way to fix this).
Twilight picked up the now comparitvely small book and flicked through it with magic. She had to start squinting to read it, but it had to have some way to help her, she couldn't stay stuck like this.... surely?
(No... I'm not going to freak out. I've done that a lot of times and its just made things worse. I'm sure I can find a way to fix it and if I can't..... then I'll panic.)

Twilight spent a good while reading the book, trying to ignore how cramped she felt before she dared to  act again. One mistake could destroy her room and cause widescale embarrassment, literally. Or some other unpleasantness she didn't want to think about...
Twilight finally found what she was looking for, a spell that would hopefully put her back to normal. She read over it twice, to make sure she had complete understanding of it and cast it.
(Here goes... something).
A purple glow surrounded Twilight and to her relief she started shrinking, her indivual changes also becoming less drastic. She smiled, and waited for the spell to restore her to normal.... only for it to soon stop abruptly.
Twilight growled in annoyance and checked the book again. As far as she could tell she hadn't done anything wrong. She tried casting the spell again, the glow appeared but nothing happened. Twilight moaned in frustration. Looking around she realised it could have been worse, she wasn't sure exactly how large she was, but estimated she was a bit large than Rarity in all directions, though her hair still hit the floor (but wasn't quite as long as before). 
(I guess this will have to do....) Twilight sighed. SHe noticed the front part of her dress was torn, so ripped a part off an older one to cover it. The patchwork stuck out, but did it s job. (I'm no Rarity. I'm going to have to see her about a replacement dress... and fix these two). Twilight thought, slinging the dresses over her shoulder, while carrying the book under her arm. 
Twilight stood up, reliefed that she no longer felt trapped inside her own room. Her head was nearer the ceiling than she would have liked, but she would have to adjust to that. Taking a few steps Twilight noticed that walking with her new size and frame felt strange and awkard. She renewed the small amount of telekinesis on her chest, but walking with the long legs felt wierd, although she was covering more distance. After a few more steps Twilight felt her balance was improving, though it would be a while before she actually felt used to this. 
As Twilight headed outside, Spike called to  her.
"What were you do- Woah!" Spike said as she saw Twilight "That's a good look for you."
"T-thanks." Twilight said a little embarrassed "And I was... uhh... helping Rarity with something. Anyway I've gotta go.. see her about something." Twilight quickly moved for the door, starting to get some use out of her longer legs.
"Anyway I-" Spike began, but Twilight was already out the door. He shrugged, stranger things had happened, often on a weekly basis.
Twilight walked briskly to Rarity's parlour. She was covering more distance than usual, though her new legs still didn't feel quite right, and she had to rebalance herself several times. Like Rarity, Twilight received numerous stares, though unlike Rarity these made Twilight feel self concious, and attempted to turn away while quickening her pace. It was then she noticed her long thick hair could be problematic, sometimes snaring around her feet. Twilight sighed and stopped, lifting her hair with her telekinesis, hovering the ends behind her in a few different directions. It actually looked fairly good floating behind her, but Twilight was simply relieved it wasn't getting in her way.
Without  much more hassle Twilight soon reached Rarity's parlour. She knocked.
"Hey its me" Twilight said.
"Come innnnnn." Rarity said enthusiastically.
Twilight did so, a little sheepishly. Rarity gasped happily. She hadn't undone her changes, but could see that Twilight was a good deal larger and went over to inspect her.
"Oh darling, you've really out done me!" Rarity congratulated, still looking around at Twilight.
"Yeah... well this isn't completely intentional." Twilight said,  embarrassed at both her own error and Rarity gawking at her. "I kind of botched the spell and ended up so large that I was stuck in my room. I tried to undo it and this is still larger than I wanted..."
"Oh..." Rarity said, looking behind Twilight. "Well I do like what you've done with your hair!"
Twilight looked over her shoulder "Yeah..." inspecting it Twilight realised it actually looked pretty good as well as being practical. "Well I had more hair than I wanted, and this seemed like the best way of managing it."
"It looks really good! You MUST show me how you did that." Rarity ran her hand through Twilight's hair "Wow its so thick... I bet I could make a lovely cape or scarf from it." 
Twilight shot Rarity an annoyed and confused look.
"O-h well I didn't think it was a big deal if we could grow our hair that long or thick, I won't mind using mine."
"Its ok." Twilight sighed. "I'm just feeling exasperated."
"Why? It looks like you had an overwhelming success!" 
"Yes, that's one way of putting it." Twilight snarked.
"Come on, don't be like that, its not so bad!" Rarity said with a wave of her hand. "You simply must show me how you did all this!"
"Don't you get it?" Twilight said angrily "This isn't magic to be taken lightly!"
"Ok! Ok! I'll be careful, just show me what you've learned and I'll fix those two dresses for you and find you something nice to wear."
Twilight reluctantly agreed. "Ok, but I mean it, be careful!"

Twilight passed onto Rarity what she knew, admitting her knowledge wasn't complete and repetitively cautioned her not to overdo it.
"How did you  avoid damaging the dress?" Rarity asked
"Well I didn't avoid it completely..." Twilight said, pointing to the patched over part of it "But I prepared a spell before hand so the dress would adjust to my body."
"Ohh thats a really good idea!" Rarity complimented "But why wern't you able to change dresses afterwards?"
"I told you I got stuck!" Twilight complained "And I don't even know why... maybe I made too great a change at once?" Twilight looked down at herself "Erm.. how are you going to find a dress that fits me?"
"Oh that's no problem at all!" Rarity walked and beckoned Twilight fourth "I had a dress that I was preparing for Princess Luna, the size should be about right for you and the colour would go great with your eyes and hair!".
Rarity lead Twilight to a large dress hung up on a special mannikin. It was a long, night coloured dress (a blend of blue and black of varying shades) designed for formal wear, with an elaborate skirt, not dissimilar to a wedding dress, and crescent moon emblem printed on the front. Twilight stared in surprise.
"Wow.. thats a lovely dress." Twilight stared. "But are you sure you don't mind giving it to me? Or that Luna wouldn't mind?"
"Not at all! This is Luna's gala dress which is several months away, so I have more than enough time to make another." Rarity said, then her toned dropped "Besides, its my fault this happened to you, and you can consider this partial payment for helping me!"
"Thank you so much." Twilight said, overwhelmed by her friend's generosity "I don't know what to say>"
"You don't need to say anything!" Rarity said, excitement building up "Just put the dress on!"
Rarity pointed to the screen she had for just this purpose and Twilight changed into the dress.
Although it was a little loose, the dress suited Twilight very nicely.
"What do you think?" Twilight asked uncertainly.
"Darling! That dress couldn't look any better on Princess Luna herself!"" Rarity said, happy for Twilight and proud of her own work.
Twilight looked down "This is definitely one of your best dresses, you must have spent a long time on it."
"Indeed I did!" Rarirty said. "So are you ready to help me practice?"
"Alright but-"
"I know! Be careful! You said so already."
"I thought it bared repeating."
Twilight went over with Rarity the spell for having the dress adjust to the wearer and what she needed to do. Rarity worked that spell, then the other.
Twilight was still wary, ready to try and dispel anything that might untold that might happen, but Rarity stared growing again in a slow and controlled fashion.
"Ha! Wonderful!" Rarity said looking down at herself.
"Don't lose focus Rarity!" Twilight said, still wary.
Twilight watched, a little envious over how much control Rarity had. She was growing a lot slower than Twilight had, but in a much more elegant fashion, each change being increased just a little at a time.
"Wow.. your really good at this." Twilight said, more envy than she wanted slipping into her voice,  surprised anyone could beat her at her special skill.
Rarity soon rose higher than Twilight, her dress growing in perfect proportion.
"Don't feel too bad, it was you who helped get me this far. I bet if the two of us worked together, we've have no problem mastering the book!" Rarity was a head and a half taller than Twilight now, and very close to the ceiling.
"Maybe your right, Rarity." Twilight said a bit happier "Shouldn't you stop that spell soon?"
"Oh right!" With a wave of her hand the spell stopped inches before Rarity hit the ceiling.
She was now slightly more than a head and a half taller, with changes similar to twilight but on a slightly greater scale. Her breasts were notably several sizes larger.
"Are those really nesscary?" Twilight asked
"Well they will attract attention." Rarity said with a smile "..to my wares of course!"
"Of course..." Twilight snarked.
"Anyyyywaayyyy..." Rarity said a little embarrassed "You could do this to, you just need to take it slowly.
"Well the other thing I'm worried about is getting stuck at a set size." Twilight said "Are you sure you could go back to normal if you wanted?"
"I think so. I guess I had better check."  Rarity concentrated, and sure enough she started to slowly shrink, soon back to looking up at Twilight. They both waited expectantly, and Rarity did reach her old size, though possibly an inch or two taller.
"I guess that answers that." Twilight said.
"Yes, but I think I prefer being how I was so..." Rarity also didn't want to admit how strange it felt having to look up at Twilight. She cast the spell and began to  grow back to her larger size.
"See Twilight?" Rarity said, slowly growing as she spoke "Its perfectly safe, you just need to be careful how you use it, that's all."
"I guess.. I feel strange asking this, but would you mind showing me how to use this magic?" Twilight asked sheepishly.
"Not at all, even the best need help sometimes."
Rarity gave Twilight a short lesson about the magic, how it was different from any of the spells Twilight was used to casting, how the changes had to be carefully sculplted, controlled and focused, lest you lose control and how important it was to keep your cool even when your not certain what.
Twilight took this on board and soon felt ready to put what she learnt into practice. She cast the spell and waited, a little nervously, but to her relief grew to match Rarity in size, stopping just before hitting the ceiling. Twilight gave a sigh of relief.
"Thank you for your help Rarity, that wasn't so bad afterall."
"Anytime!" Rarity said with a smile. Twilight took a few steps. 
"But this still feels strange to me, I think I'm going to shrink down a little." Twilight did so, approaching the height she was when entered Rarity's parlour.
"So do you have any particular plans for you dresses?" Twilight asked. Rarity scratched her chin and thought about it.

"Well I have a few I plans to show off my dresses and I believe this magic might help with it, but not many I'd specifically need it for. Although I think this alone opens up another possiblity.... "
"What do you mean?" Twilight wondered.
"Maternity dresses!" Rarity said excitedly.
"What?" Twilight asked, puzzled by the concept 
"Well even mothers to be  want to look good! And um.. some people just like the look."
"Okk.." Twilight was even more puzzled, but trusted Rarity understood these things.
"And if we can make the dresses adjust to the users body." Rarity continued "Then every dress can be a maternity dress!" 
"Interesting." Twilight mused "I've wondered if spells could be sewed into clothing..."
"Of course! Though first we have to practise..."
"Belly expansion.' Twilight flicked through the book. "I'm surprised that's in here..."
"Well the people of old did appreciate wide hips and a fertile woman."
"I suppose. I do feel a bit nervous about trying this.." Twilight confessed
"Don't worry I'm here to help. Just take it slowly." Rarity reassured.
"What if I mess this up and end up with a huge belly that pins me to the floor?" Twilight started panicking.
"Well you could just reverse it."
"What if I get stuck with it?!"
"Just take it slow, don't do more than you need to." Rarity touched Twilight's arm reassuringly "Your magic is powerful, but this requires a gentle approach. Just do a little then I'll give it a shot."
"O-ok" Twilight said, a little more assured.
Twilight concentrated and cast her spell. A warm purple glow briefly encircled her stomach then vanished. 
After a few seconds her stomach started to swell, as though she were pregnant.
"Its working!" Rarity squealed in joy, then got serious "Ok Twilight keep focused, picture the belly you want and sculpt it on."
Twilight nodded, concentrating as hard as she could. The belly grew at a steady yet controlled rate, with Twilight going through months of pregnancy in a matter of minutes. She stopped around the six month range, her skirt having a large, but not unflattering bump in it.
'Fabulous!" Rarity called out, hugging the new belly. "Ohh it feels so soft!"
"Erm... I'm glad you like it?" Twilight said awkwardly.
" I can't wait to try this out!"
Twilight showered Rarity how to cast the spell and she eagerly did so. Her belly rise up slower than Twilight's, but she eagerly urged it along until it soon surpassed Twilight's by a good margin.
She playfully bumped her belly against her friend's.
"Look Twilight twins!" Rarity said triumphantly.
"Yes I can see that Rarity... good job."
Twilight noticed Rarity's belly was still expanding.
"Shouldn't you do something about that?"
"O-oh right!" Rarity said, stopping the spell. 
"Sorry I just got caught up in the moment!" Rarity said, hugging her belly. It was so large Rarity had trouble getting her arms around it.

"Wonderful!" Rarity declared. "Now when things are going so well, would you mind showing me what you did with your hair?"
"I just put more energy into the spell than I should, and let the spell run longer than I intended, this wasn't suppose to happen." Twilight sighed.
"Oh well that's a shame, I really like your ma- hair."
"Thanks I guess..."
"And this makes the answer really simple!" Rarity said, focusing hard before casting another spell. Her hair start lengthening from all directions, quicker than before.
"Rarity are you sure this a good idea?" Twilight asked in a concerned tone  "If you cast that spell carelessly you could end up with much too much hare!"
Rarity merely smirked as her hair began to spread across the floor in all directions. She didn't seem to be trying to reign it in or sculpt it. 
"How much hair is too much hair anyway?" She asked in an amused tone.
Rarity let in continue to grow, spreading all over her parlour, and around Twilight, while lying against it and enjoying its softness.
"I do like how clean it looks to!"
"I think you should stop soon..."
Rarity's hair was close to filling half the parlour.
"Yes, this does seem like a good place." She smiled and suspended the spell. Rarity walked through the parlour, savouring her new hair.
"Isn't that an awful lot of hair?"
"Doesn't it look gorgeous? I am going to have to levitate it of course, but it will be totally worth it!" Rarity said excitedly.
"I guess it will help you get attention..."
"Exactly!" Rarity gestured excitedly, still walking around enjoying her changes, then she suddenly she turned around and became serious. 
"Now, theirs one final change I need your help with, and I think this one might be the trickiest."
"I'm listening." Twilight said with some unease.
"W-w I noticed that we could increase our heights with this magic, and I figured, what's a better way to advertise than displaying my wares for all of Ponyville to see!"
"What?!" Twilight said in disbelief "You want to use this magic to become a giant?! Just to show off your clothes!"
Rarity grinned "Well I always think  a giant me would make an impressive sight, don't do you?"
"Are you crazy!? Think how risky this could be, and for such a frivolous reason!" Twilight yelled angrily.
"Frivolous!?" Rarity said angrily, pushing against Twilight's face with her own "I'll have you know fashion is my life!"
"I'm sorry.." Twilight's tone softened  "But is it really worth such a great risk?"
"What risk?" Rarity countered with a wave of her hand "You said you were apparently large enough to be stuck in your room."
"Not by choice!" Twilight complained "I was STUCK."
"But you got back to normal.. relatively speaking."
Twilight sighed "Yeah.. but I don't know what I would have done if I couldn't."
"But it seems like you could avoid any side effects just by not biting off more than you can chew." Rarity said hopefully.
"That is possible.. but on the other hand any possible side effects could be much worse."
"Well how about you take it slow and if you feel at all uncomfortable, stop immediately?" Rarity suggested.
"I don't know... it still sounds risky."
"What if I double your payment?" Rarity said leaning forward "Can you really say no to more books?"
"M-maybe."
"Plllleasseeeeee twilight! I'll be super careful! Pretty pleaseeeee!" Rarity said, staring passiontly into Twilight's eyes, leaning closer than normally considered comfortable.
Twilight sighed. "Alright, but you better not make me regret this..."
"Thank you, thank you, thank you!" Rarity said, squeezing Twilight for a tight hug. Twilight was glomped.
Some people would call this a marshmallow heaven, for more than one reason. Twilight wouldn't.
"Umpph! Rarity... that's a bit tight." Twilight gasped.
"Oh sorry" She said letting go. Twilight took several deep breaths.
"Anyway lets go outside!" Rarity said, not noticing her friend's problem.
"Give me a bit..."

Although she had (inadvertently) done this before, Twilight insisted reading this section of the book several times before she even though of trying it again. And naturally it was the most complicated. When the mage finally felt ready, the two of them went outside.
"I'm still not sure I want everyone seeing me like this...."
"Come on Twilight, you look wonderful in that dress. And I'll happily join you if this goes well." Rarity said "Also, think books!"
"Ok, ok." Twilight sighed.
(If I start with my previous size and I do this carefully, I hopefully shouldn't have too many problems. And if I do, hopefully I'll be able to call the spell off before it gets problematic.) Twilight planned, inadvertently picturing herself growing larger than Ponyville.
(I hope THAT never happens.) She thought with a shudder.
Twilight cleared her mind of such distractions and cast her spell, She concentrated as hard as she could, focusing on the spell, the growth, and keeping her mind free from panic. After the usual delay, Twilight felt herself growing, her body filling with energy. It was a little slower than before, but Twilight felt she had much better control over it this time. To her relief, the dress was able to  match the pace of her growth perfectly.
"Its working! Its working!" Rarity cried happily.
"Rarity please! I need to concentrate."
Twilight noticed everything getting smaller, and she quickly surpassed Rarity's height. She stared up at her in delight. Twilight grew in a careful, ordered fashion, stopping at around the height of Rarity's fashion parlour, keeping the same proportions she had before. Twilight breathed a sigh of relief  that nothing had gone wrong, though standing and looking at the diminished Ponyville felt strange to her.
"Well Twilight? How do you feel?" Rarity shouted up to her.
"I can hear you just fine." Twilight said, her voice sounding considerably louder.
"So no side effects?"
"None that I'm aware of." Twilight said still feeling strange.
"Except that..." Looking around she could she everyone around was staring at her. Some shocked or scared, others... admiringly? Did she look that good like this? She blushed.
Rarity saw the crowd to and stared enviously "Oh darling that's not a bug, its a feature!"
Twilight looked unamused.
"Don't be like that. And show me please, I want to get in on this!" Rarity said , stamping her foot impatiently. 
Twilight bent, minding Rarity as she lowered herself. She showed Rarity the workings of the spell.
"And please, PLEASE be careful, you don't want this spell going wrong in front of al of Ponyville."
Rarity flushed awkwardly imagining the worst case scenario, then cleared her head and prepared herself.
"Okkk here goes!"
Rarity cast the spell and rapidly started growing. Her speed was a lot faster than Twilight's, because while Twilight has been holding herself back for fear something would go wrong, Rarity just wanted to catch up with her friend as quickly as possible.
Rarity smiled as the crowds eyes were drawn to her growing body, and it helped that she kept the larger proportions, her hair especially covering an increasingly wide area.
Before too long she had caught up with, then exceeded Twilight's size. Standing behind Twilight it was noticeable that Twilight only came below her breasts, and Rarity's height was still increasing. Twilight stared up in shock.
Her smile turning to a crowd, Rarity stepped out to greet their audience.
"Thank you, thank you!" She gestured extravagantly to the crowd. "Welcome to Rarity's impromptu fashion display!" 
Rarity loved what her size added to her showmanship voice. An increasing amount of eyes we're on her, and she was loving
"Are you sure this is the best way of doing this?" Twilight said looking up to her "I thought you we're going to show a range of dresses?"
"W-well yes, but what's a better way of setting up for that than with some advertisement? " Rarity said defensively.
Twilight had to crane her neck higher as Rarity continued grew "Are you sure your not just enjoying the attention?" 
"Can't I do both?"
Twilight sighed.
"Ok, ok, I do have a few dresses prepared. Check the right corner. I think the studded dress should go over particularly  well. I'll keep the crowd occupied.
"Of course." Twilight said disdainfully, then started shrinking down to normal size.
"Don't worry folks, she'll be back in a minute, she's just getting another of my fabulous dresses." Rarity wondered if she were starting to overuse the word fabulous.
Twilight looked at the dresses. They were all meticulously crafted, which was standard for Rarity, but the one Rarity had indicated stood out right away, even from a distance it shone brightly.
As  Twilight approached it, she saw that it was a light blue dress, laced with gemstones and diamonds. Had they been carelessly thrown on, the dress would  look very tacky indeed, but the gems were arranged with precision and care, each row having a different set of coloured gems, with each set bright enough to be noticeable without being garish, with a few diamond in a horizontal line on the dress, albeit more spaced out than the other gems, and with enough space overall to show the dress itself, the gems enhancing the dress rather than distracting from it. Twilight held it up and took a few seconds to admire it.
(Is she getting better at this?) Twilight wondered. Rarity was always a good dress maker, but what she had seen so far exceeded her expectations.
Twilight put the dress on and rejoined Rarity. Stepping outside she was astonished at how large Rarity grown. Twilight came well below her knees, and her hair rolled all the way along the street. Had Rarity made it even longer?
"DON'T YOU THINK YOUR GETTING TOO LARGE?" Twilight shouted up to Rarity.
"Not at all." Rarity said looking over her shoulder "In fact, I think Equestria will remember this as one of the greatest marketing campaigns of all time!"
"WELL JUST BE CAREFUL NOT TO BREAK ANYTHING."
"Of course I will! And you don't need to shout, I can hear you just fine. Anyway come on and join me, so the crowd can see your dress."
Twilight cast the spell again, this time aiming to quickly grow near Rarity's height, the gems thankfully growing with her.
"Now keep the crowd occupied while I get another dress" Rarity said, slowly shrinking down.
Twilight would soon be higher than Rarity  "But what do I do?"
"Just let them see your lovely outfit! Those gems should reflect quite nicely."
"O-ok." She waved self consciously to the crowd and twirled around her skirt.
(I hope this gets me a lot of books....).
Twilight was fast approaching Rarity's old height. She continued to pose with a nervous smile on her face, most of the crowd were staring suitably, which was good in a sense, except it was making Twilight even more nervous. And she soon realised she had exceed Rarity's height, but was still growing.
(Oh crap! Terminate spell! Terminate spell!) Twilight thought, pointing at herself and trying to halt it.