Looking at the patchnotes, it seems the max limit code was edited in 1.5, I'm wondering if maybe something went wrong there and the game is misinterpreting it as not being full enough. I've also had my belly size randomly rank up despite not being big enough
Viewing post in Yuki Pop Repeat~ (Ver 0.1.6) comments
Honestly I'm not entirely sure how the max-rank-up thing works, but from what I can tell, you can either focus on belly to boost just that way beyond the usual for a bloat run, the other two for an hourglass run, all three equally to just push from tier B to A normally, but after reaching either bloated, hourglass, or A tier elasticity, you can't go any further without popping unless you happen to have any cowbell item in your inventory, which can turn A tier into "Cowloon", which is the biggest tier I've found (albeit it has the drawback of requiring the lactation curse to be applied which typically means imminent popping). I kinda wish there was an S tier or something like that, but I don't think there is.