ok so stormtroopers obviously have to be incredibly physically fit and everything. even after finn left the first order he kept his training schedule (at first only easy stuff so he doesn't aggravate his wounds, but later on he does some intense workout) and obviously poe is all over that (he has a huge thing for muscles ofc) while he himself is more on the chubby side (think oscar in inside llewyn davis)
he's not exactly self-conscious about it, but he doesn't consider it a big deal, nuuut to everyone's (nobody's) surprise finn turns out to like poe's little tummy. very much.
Bonus: if maybe finn's tummy!kink comes from him associating it with softness and kindness, something which the first order didn't tolerate. also if you want them to do the do, i would prefer bottom!poe :)