Currently Thinking About Mutual Gaining Spa/mtenna.
I haven't thought of a good enough scenario to really write a fic or anything but here's a few little headcanons about their gaining:
--Spam, especially in a scenario where they've reconciled and started dating again, would be less bothered about gaining weight. He'd be almost proud when he notices he's getting pudgy...he can afford to eat well again, and that's worth showing off!
--Spam is also someone who will eat like a starving animal (because he *was* one), so fast he won't even notice how full he is until his pants button is threatening to burst. Fortunately, Te/nna's large hands are excellent for giving him belly rubs.
--His fat settles mostly on his middle. He just becomes a round little ball, like a squishy lump of dough, and Te/nna can practically roll him around (though he hates it).
--By contrast, Te/nna would be an insecure bundle of nerves when he starts gaining weight. This guy hates change, and he would not initially react well to his body changing, forcing his pants on even though they're clearly too small, trying to suck it in or wear a girdle. Spam has to constantly yell at him to let it out and comfort him when he starts sobbing because he ruined his pants and can no longer deny the fact that he's getting chubby.
--But eventually, he sees how much Spam is enjoying his weight gain...how he seems even more attracted to him than before. How he's giving him extra attention and always making sure he gets enough to eat, despite it taking a lot to fill him up. And then he starts showing off a little for him...purposely stuffing himself just to get Spam all excited. Making a private little show of it. And maybe he's starting to enjoy it for himself too....
--Though he still has a belly, Te/nna would be more bottom-heavy than Spam. He's already got dat booty, and it's just gonna get *bigger*, lol. Spam loves nestling into his bigger, softer lap like a weird little cat.