Imagine person A of your OTP is underweight, and wants to gain more weight, enough to get fat. But, A’s appetite is so small, they can’t eat more than a few bites without feeling sick. Person B has a very big appetite, and loves eating, but B doesn’t want to gain weight, so makes themselves eat less than they’d like to. Through some magical/sci-fi means, A and B switch bodies, so B can can gorge themselves in A’s body, letting A get fat and B eat however much they want without gaining weight.
Days pass, B not doing much but eating and laying around, stuffing A’s stomach until it’s round and bulging, aching with fullness, but B’s enjoying eating too much to care about it hurting. A’s body starts putting on weight, B sending A photos every so often, along with a log of how many pounds they’ve put on, and A keeps coming back and telling B to put on more. As A’s body gets fatter, B finds themselves kind of enjoying the extra weight, playing with A’s growing belly and thighs, tempted to touch A’s plump butt and chest, but feeling that wouldn’t be appropriate.
Eventually, A and B switch back, A delighted with their new, fatter body, but B kind of misses getting to play with all that chub. B admits this, and A bashfully tells B that they can keep touching if they want. B starts rubbing A’s soft, round tummy, hands getting a bit rougher as B’s touches turn into gropes and squeezes. A lets out a few small noises, gasping and biting back moans, squirming closer to B and encouraging them to touch more.
It only seems natural for them to kiss at this point, B’s hands wandering all over A’s soft body, squeezing their thick thighs, groping their cute, plump butt, massaging their swollen chest. A keeps pulling B closer, until they’re pressed tightly together, moaning into each others’ mouths, and A’s clothes are already bursting off of them at this point so it doesn’t take much effort to pull them off as they tumble into bed together.
After several rounds of rolling in the sheets, they’re laying together, panting, sweaty, and very pleased with themselves, B’s head comfortably resting on A’s cushy chest. A’s tummy starts rumbling, and they chuckle and apologize for ruining the moment, saying B’s gorging must have forced their small appetite to grow. B simply gets up, raids the kitchen, and then proceeds to stuff A senseless until they can’t even sit up on the bed. Too full to think straight, A drifts off into a food coma, smiling sleepily as B rubs their huge, gurgling belly, leaving sweet kisses on A’s plump cheeks.