It seems that Rarity is starting a little club to counteract Pinkie's obsession with cramming cupcakes down their throats. Whoever comes out on top in this war of sweat and sweets...
You win.
 
Rarity's body vs Pinkie's sweets. Was there ever any doubt?
 
Rarity has been trying admirably to contain her fat within a range. Thin is in, but fat is still acceptable. Being a blob is simply horrible. She's a determined mare when she wants, so she's fighting well. See the picture of her trotting her hefty haunches off? That was taken by Sweetie Belle trying to get her photography cutie mark.
Rainbow Dash is probably the most fit pony, and has burned off the most weight since entering. Of course, it fluctuates pretty wildly with her swings into sloth. Pinkie is particularly interested in her friend's progress, as it's the most chaotic and fun to watch. She's merely quite chubby as of now.
Applejack actually has mixed feelings about this. Normally balancing her heavy workload with large country meals, she's kept a strong, sleek profile. But Pinkie's interference has turned her job upside down. In addition to the flab coating her body, her muscles have exercised constantly with the increased effort of hauling everything. She's strongfat, with simply massive bucking haunches whose mass and strength have made her job actually easier. She's just a tad larger than Rarity, and Pinkie is just a talk away from convincing her to give in.
Twilight started out the largest of them all. A sedentary lifestyle and being used to Canterlot luxury left her wide open for Pinkie's deluge of candy and cakes. She plumped out mighty fast, and it was only when she realized that her walking was knocking over fully shelved books that she had to do something about it. Armed with knowledge but hamstringed by her desires, she's worked off a little weight. Her star-covered ass is still large enough to make everypony uncomfortable... or intimidated.
Now, who would Pinkie go for the most? The fighter, the friend, the uncertain, or the biggest?
 
Dashie the Friend.
 
And as Pinkie, watching her relatively small belly and tight plot rush about during exercises, what would you do to get her back up to a snuggleable weight?
 
I would convince her of a new work out technique that requires gaining weight then shedding it off. however, it has to be a lot or else it won't work.
 
And with her experience with the quick shift of her extremes of weight, she'd likely believe you. But how would you get her the information? They know you're the source of the extra thousands of calories.
 
I tell her that it would maximize her chances with the Wonderbolts. This diet would be the one key needed to achieve her dreams. "And Dashie? I would wannnnn- WOULDN'T want you have... the chance of your life to pass you by!" The grin came naturally for her.
 
Rainbow Dash would look at you exasperatedly at first. You are the reason, after all, that she's so fat. Even when you reassure her of how much you care for her and never want to see her unhappy, there's a glint of mistrust in her eyes.
"Pinkie, what's this all about? I don't need a diet; I'm losing weight just fine!"
 
Pinkie frowns, the turns her head a bit before looking back a Rainbow with puppy dog eyes.
"Dashie, all you had to do was gain some weight then lose it really fast that way your body would be able toget used to quick conditioning."
She sigh dragging her forehooves along the ground, "if you want to do it your way that is fine to, I just wanted to have fun helping you be the very best flyer in all of Equestria. So no pony could ever dream of keeping up with you amazing, breathtaking super-duper ultra mega," she takes a deep breath, "SPEED!" She screamed out the last word. Pinkie got her own motor going and she already bouncing around with a smile on her face.
 
Dash looks far off in the distance, as if sizing up an ornery cloud. Slowly though, her face softens and relaxes. With a worn hoof she scrapes at the ground. "I guess... I could try it. That's the point of the whole exercise group after all. Trying things to lose weight." Her thoughts seem to hit a brick wall though, and she recoils suddenly.
 
"Pinkie... I need you to promise something. Until I get back to normal, and I mean normal-normal, I need you to not breath a word of this to the others. They wouldn't get it. Alright? Just, you're still my friend. Say I caught a really nasty case of the Feather Flu.
 
"Okie doki loki!" She stated with the up most confidence, RD could only feel reassured.
 
Well, obviously the others would send their condolances, as would most of the weather team.
 
Pinkie would keep them up to date with Dashie's well being.
 
It would be pretty easy for you to slip a few candy bars into their cards. After all, anything for a friend.
 
Maybe... no definitely with extra sugar.
 
So how does Pinkie make sure her Dashie gains just enough to prevent her from losing it all in a single burst? Too much, an immobilization, and she'll know the jig is up.
 
Easy, We go in progressive bursts at first, ending each time with her best treats for last; then, that would leaving him wanting for more. After a bit, like a few weeks, I would kick it into high gear, I would tell her it is the final step for her to achieve her goal.
 
An alibi of sickness only covers a week or so, maybe three meetings. Too long and her friends will call a doctor. Deep down you know you'd do the same thing: you care too much for everypony.
 
Then that would leave her with one/few option(s), so she would...
 
Do you do an accelerated regimen, or try to reduce her permanent gains? Of course, being Dash, the "diet" works for her. She's enthusiastic, but uncertain if it will just bounce her around more or give her results.
 
Pinkie would assure Dashie she would get the cream of the crop of results. Then she would have to ask of a favor of her soon to be Wonderbolt friend. She would tell her to stay hidden so she would be able to impress everyone with her new found talent.
"I'm not going to lock you away, but if you are going to play sick, I need to make sure they will believe it!"
Of course Pinkie would tell her friends, after a while, that Dashie has been preparing a secret, ultra-special trick for the Wonderbolts and it need to stay to secret.
 
Rarity would be the most angry about it. "That rainbow-maned foal is not supposed to be slacking off on us! She's the Element of Loyalty!"
Applejack would also be wary, but more accepting of her friend's absence. "It's not the end of the world, Rares. She worked off most her weight anyway. She can stand to practice her passions."
Twilight would be happy that her friend is back to "normal" and flying. "Yes Rarity, it takes over 10,000 hours of dedicated practice to learn mastery of a single skill. As much as Rainbow Dash has flown her whole life, an original trick is something that takes time to do.
 
Pinkie would hype Rarity up with her, reassuring her that everything would go as smooth as a fresh cake with nothing on it. Then ponder and shrug off the odd expression as she gave Rarity another beaming smile.
 
Alright. You got them.
But now that you've gotten enough time, what do you do with it?
Dash is currently at a low point, zipping about and putting on a show for anyone who cares to look. A good compliment to your alibi.
But soon enough she'll wear out and you'll be able to give her another push towards the soft side.
 
Pinkie then goes and fetches a bakers dozen of cupcakes and cookies with a sizable glass of chocolate milk. She place the food in from of Dashie and allows the freshly backed smell lure him into diving in. Pinkie watches with a big, coy grin. Bobbing her head side to side like an imaginary pendulum dancing to music.
 
Rainbow Dash watches you with obvious amusement, she sighs. Her belly is almost back to it's normal sleek size, to your disappointment, but entire nights of binge eating have stretched her stomach enough that you doubt you'll have any trouble stuffing her full. "Just one more. Right? Just one more..."
Dash never liked to eat past her previous limits, so whenever she stopped you insisted on another plate or the next treat. All hoof-fed of course.
 
Pinkie takes a few of the cookies and places them in her mouth and waits for her to swallow before placing more. Pinkie subconsciously shifts into assembly line mode as the cool down time in between placing of sweets withers into rapid feeding. She takes a quick look at Rainbow before her eyes wonder toward her belly, her vision snared and unable to looks away she keeps packing away the sweets until she out of ammo. Remorsefully, she glances at Dashie in the eyes and give her a hug. Secretly, she rub the soon to be grounded athlete's belly, feeling the partially gained softness mixed with the taut feeling of being full. She lets go, "I think we can go for a bit longer. If that is okay with the greatest most fastest pony in all of Equestria."
 
As you push each dish into her gut, you notice the slowed chewing and heavy breathing as more and more of her hangs lower. In the end, Dash groans as you squeeze her churning body. Even though her stomach is taut and swollen with pastries, it's nothing too serious. She's gotten bigger before, back when you accidentally cooked her favorite cakes and she demanded one after another. Her eyes don't focus for a second until they catch your ecstatic stare. "I c'n take it.." She gives you a half-hearted smile, her cheeks slightly chubbier than they were an hour ago. "I'm awesome."
 
Pinkie dashed for the kitchen and funneled her energy into collecting an arrangements of sweets from flan, to pie, to gingerbread houses (and any combo of sweets). She didn't waist a second and feeding them to Rainbow. A quarter after her start time she began to go into a full stuffing; the baker-feedie relationship Pinkie had sought has came into fruition and all rationalizing was discarded. No regards if her plump friend tasted it or not was given. One after another and then some more, a cycle of feeding and, what Pinkie convinced herself of believing, endless appetite. Out of ammo once again Pinkie though to herself that she need to go big this time.
 
Pinkie made a mad dash for the kitchen and whipped up something to make a nice finish. After what seemed like endless moments, she returned with the most fattening cake she could think of. It was a three story cake with several layers and a different filling for each one. The frosting was done in a rainbow of colors and candy -- and sprinkles and other sprinkle like confections, were scattered all over. The cake its self was dipped in chocolate too. Last but not least, the cake was overdone in in sugar while creating it making the sweets treat around.
 
She started to feed RD without even asking if she had enough. Smiling as she stuffed several slices at a time.
 
Dash, watching Pinkie roll in that monster of a confection, wondered just how in Tartarus she managed to stuff so much unrelated sugar and junk food into a single object and still have it look as... as wonderful as it did. Her stomach growled in protest as she stared, sending a telling echo throughout the room. Almost pained by the betrayal of her own body, she whipped her gaze between you and your beast.
Opening her mouth to raise a concern, she was met with a flurry of sweet bread, fudge, candy, frosting, and... was that pure sugar? Dash shook in shock and confusion as she instinctively chewed at the decadence. She barely choked down her mouthful before you shoved another battery of caramel and jelly filling. Slice after slice of goopy, dripping food was shoveled into her, and her gut started to bloat beyond her expectations.
 
"Soon you going to be a champ, and you will never feel the same. I bet you will be filled with a great feeling of joy, and you will have your buddy Pinkie to thanks for all of that 'inner' joy!
She didn't miss a beat, borderline shoving it in her mouth letting. The gleeful baker made sure that Dashie chewed and swallowed every load; if you put all this meticulous work into cake of this magnitude and sugary goodness, you would want them to taste ever drool generating bite.  Pinkie gave another stroke to her star's bloating gut and watched the ripples echo across her belly.
"You know Dashie, it is going to be a lot of work to burn all of this winner's fuel. How about you come and crash at my place. We can have a sleepover! Doesn't that sound like the BEST THING EVER!."
She continued to feed Rainbow, and watched her labor be put into effect. She watch as fold formed and cheeks pudged out a bit. Time would need to take effect before she could enjoy her marshmallow friend in the making. After the cake was consumed, she gave a walk around Dash scanning over her expanded frame. She prodded a few part and observed how far her hooves dug in.
"Eeeehehe", Ms. Pie squealed. "Would there be anything else you would like to be stu-- eat in preparation!" She put on a facade of reassurance hoping to bait RD into taking seconds. Her heart's desire was none other than to see her friend become a star: her own big star.
 
Grounded by her own bloated stomach, Dash groaned as Pinkie paced around her pudge. The meal before that cake had been huge, but actually having that monolith of sugar and fat forced into her was nothing short of a miracle. She sluggishly reached a hoof down her gut and massaged futilely at the groaning mass her body was attached to. It felt hard and immensely uncomfortable, but gave just enough under her own touch to feel strangely pleasurable. Each rub caused a cascade of inner gurgles, gas shifting and rearranging itself in her body. With a stifled burp that tasted strongly of chocolate and toffee, Dash managed to say, "Pinkie, could I ju-UUUuuurp... just get a glass of water?"
 
Pinkie smiled, "I can get you more than a glass of water! Be right back in the quickest second before you can even say 'Gee Pinkie that was fast! how did you do that?'
Pinkie shot out of the room and made way for her kitchen where she took a few gallons of sugary sweets and mixed them into a single drink. A chemistry experiment of chocolate milk, sodas, with ice cream on top. Restraint was needed to not drink it while she was making it. With prefect balance she present the drink before Dashie. "Drink up!" The drink was no small thing. It was packing thousands on thousands of calories and was at least several liters.
 
Closing her eyes and forcing back a wave of dizziness, the cyan pony grimaced and leaned forward, placing her lips on the chilled rim of the mug. In a flash, the entire thing was upturned and rushed down into Dash's waiting throat. A blur of flavor and texture slid over her tongue: creamy, tart, sickly sweet, and back to creamy in a seemlingly endless cycle of that fattening sugar-shake. Even in her distended gut, she could feel it soak into the bread and cookies slowly being digested, saturating them with even more calories to be poured onto her frame. Her gut may have grown yet larger, but it softened it's edges. It was sloshier and jigglier, sending off waves as Dash shuffled under the torrent of sweet above her.
When the merciful end came, she could barely budge. Even if her limbs could walk without smacking into her low hanging form, the sensations of being so... FULL, just overrode any other thought that tried to get her to speak or move. Instead she flopped to her side, each breath forcing her wide gut a bit higher off the floor. With glee, Pinkie noticed how big she'd grown. The stomach, packed to the brim with all those desserts, groaned and churned with it's top almost twice as high as any other part of Rainbow's body. "Pingee... Pink... plea-EEARP-se stop..."
 
"But Rainbow, I thought you wanted to be in the Wonderbolts? You can't if you don't eat!"
She goes to the kitchen to get another over-the-top confection and returns with a bad of candy. Each piece is made of chocolate, with a caramel coating and fudge filling. The size of ping-pongs ball, each was a tasty treat. She had to have at least a hundred in that bag.
"Time for more!" a manic grin grew on her face.
She slowly pushed one into the immobile pony. Soon she put another one and then another.
 
Each little burst of caloric joy brought nothing but groans as Rainbow Dash slipped farther and farther into a food coma. Even with a stomach full to bursting and the promise to spread it all over her body as thick rolls, she chewed like a machine. Each little morsel stuffed into her maw was succinctly tasted, gnawed on a bit, and eaten.
 
Pinkie then get up on Rainbow and find her self confortable on the spread of fat. She slowly lays down, feeling the fat ripple travel a longer her custom bed. Before nodding off she said,
 
"I love you Rainbow Flab~" Then she nodded off with dreams of the next day.