There's that, but I also thought it was weird that even if you have thickness and that men store weight in their bellies, that when you're really thick, belly weight in the codex is still 0.0
From what I've experimented, ingame it's mostly just thickness, and not much to do with actually getting fat and the most you lose is muscle tone, especially with belly weight or anything. Of course it isn't that important, it's just for fun.
I feel like if you drink a lot of beer, and often, you should get one hell of a gut/beer belly. It would be a fun modification, instead of having temporary enlarged bellies like impregnation or cum inflation.