Twi, as an attempt at a RD/Pinkie style prank, transforms into a slime version of herself and makes the 'important test' of shoving herself down Rarity's throat. However she misread the effects on the eater and now, in addition to being more domineering in general, the originally protesting Rarity enjoys the feeling and is enticed by the possible weight gain if she can keep Twi down.

“Twilight, darling, what are you doi—mmff!”

Rarity’s eyes widened as Twilight’s oozing form forced her mouth open wide. Her jaws worked frantically, but in vain. She could scarcely breathe; her cheeks were bulging as Twilight’s gelatinous mass filled them; her throat was being crammed as full as it could possibly manage.

“Ooh,” Rarity moaned, gurgling slightly around Twilight’s form. The first of her friend’s mass had hit her belly, and in that instant, the sensation of panic flickered into something resembling pleasure. “Mm…” More and more of Twilight slid down Rarity’s gullet, more and more bright purple slime filling her belly by the second. Rarity had one hand on her stomach, and she could actually feel it expanding as Twilight slid inside of her. Whyever her friend had decided to feed herself to her, Rarity decided, could well enough wait. This feeling was too good to resist.

Choking turned to eager gulping. Rarity’s belly was now protruding out in front of her by a few inches, and had pushed her shirt up just as much. Her belt was feeling incredibly tight, and she was sure she’d heard her pants splitting as her thighs thickened and widened. But she was too far into it now to worry about something as meaningless as a loss of her skinny figure. No; Rarity was more than happy to swallow Twilight down whole, ready to transform her friend’s oozing mass into her own tight, bouncing bulk.

Rarity moaned as Twilight entered her up to the waist. As each part of Twilight entered her stomach, Rarity could feel her entire body expanding. A loud rip tore through the air as the inseam of her pants simply gave out. She could feel her belt trembling, quivering, cutting deep into her ballooning flesh. Even her breasts began to expand as well, gaining more and more size and softness as they pushed against her bra and shirt. Buttons popped, one by one, as it became absolutely clear that her gut simply could not be contained.

Her swollen belly as more incredibly tight, now, having swallowed up a good half of Twilight’s mass. But still, Rarity gulped on, happily taking more and more of Twilight’s body down her throat as her stomach bloated outward. She felt her stomach flop down onto her thighs as the last of her buttons burst; somehow, though, her belt was holding on. She groaned, twisted as Twilight entered her waiting maw up to her chest. Twilight’s own substantial chest oozed down Rarity’s waiting throat, hit her stomach. Rarity’s rounded, overstuffed belly strained against the leather belt, pushing as hard as anything.

With a crack like a whip, the belt exploded off of her. Rarity’s belly flopped enormously onto her lap in the same instant that Twilight’s shoulders went down her throat. Rarity’s enormous gut was jiggling and shaking from the shock, little waves of flab rippling all across her body. Her thunder thighs were rubbing against one another now; her massive boobs were spilling out around her bra. Had she been standing up, her belly would have likely hung down past her waist, if not to her knees.

Rarity’s chubby cheeks puffed out as Twilight forced the last of herself inside. Her double chin jiggled and shook along with her belly. Rarity packed on more and more pounds, her corpulent figure swelling into elephantine size while her massive legs exploded from her pants. She moaned aloud, her incredible, plump ass spilling out from the back of her once-loose panties.

And then she gulped for the final time, and all of Twilight was in her belly. Something sloshed in her great, bountiful gut, and Rarity took hold of it with all of her sausage-thick fingers. It squished easily beneath her grasp, overflowing her hands.

“Okay, Rarity,” she heard Twilight say. Her voice sounded incredibly strange echoing against the insides of Rarity’s belly like that. Rarity shivered with pleasure. Those vibrations were...incredibly erotic, somehow. They danced across her gut, tickling her, even as Twilight’s oozing form squirmed inside of her. “Looks like that worked. Time to get out of here.”

Rarity eyed her new corpulent form. Her hands roved across her body, exploring each fold with new interest. She loved how tight the remains of her pants were against her ass; she loved watching her boobs jiggle when she squeezed them against her incredibly tight bra. She gave her thick love handles a squeeze and absolutely melted with pleasure.

She licked her lips and slurped up the last of Twilight’s gelatinous mass. “Mmf,” she murmured, softly stroking her gigantic gut. “I’m positively huge.”

“Hey, Rarity?” Something shifted, sloshing, in her gut. “Can you let me out now?”

Rarity’s smirk became positively devilish. “Sorry, darling,” she said. “But I just love how full you’ve made me.”

“Rarity!”