Here’s my rules (for Pathfinder 1E, D20 3.0 and 3.5).
You’ll notice I take a different approach to the topic. I focus more on the mechanics for being obese and how existing magic and rules would apply, and keeping the introduction of new rules to a minimum, but so far don’t have a lot of content for actually getting fat.
I’m still working on ways to actually get fat, but most of what I have so far is focused on magical weight gain. I talk a little about natural weight gain/loss via food & diet, but I don’t go into too much detail since that method is usually far too slow for the average D&D adventure - and the fact I’m not real big into stuffing.