Oddly enough, RimRound and StarPounds, the fat mods for Rimworld and Starbound, respectively, are the games that have the biggest focus on fitness that I can think of. RimRound in particular is interesting as your units lose a lot of mobility at higher weights, and generally receive debuffs, which makes it an interesting challenge to be able to support a higher weight pawn for effectively no benefit from a gameplay perspective alone, leaving aside the fetish implications. Also… I know OP has seen it, but I am obligated to shill my own seldom-updated game that is somewhat relevant.
I am hopeful other people might respond, because this genre of fetish game activates so many of my neurons. God bless you for making the topic.
That dev actually has another game I am more fond of, even if it has its flaws: Week of the Werefatty. I really wish they would revisit it, although as-is, it has the curious trait that solving puzzles correctly locks you out of the fetish content, which is a shame, because this is one of the few text-adventures that has puzzles I enjoyed bumbling through. If they swapped the incentive structure, so that solving the puzzles unlocked the fat content, it would be very good, especially because currently, failing puzzles makes you fatter, which makes puzzles harder to complete due to lowered fitness, raised weight, etc. I like that mechanic, and the descriptions. If only they were won by succeeding at puzzles, instead of given as a reward for failing to engage with the puzzles!