a is only a bit chubby, not really fat, just a small soft belly with a small overhang

b notices a's chubby existence... and slowly starts giving them food, a just thinking they're being nice. but in reality, they're fattening them up even more...

during this all, b resists the urge to just hug a's belly, rubbing it when they were full, teasing them, and wanting to admit how beautiful they find them

one day, b finally just goes for it, hugging a and telling them how beautiful their belly is, how they love the fat

and then it goes from there

any ship