My sims gain weight. A lot of it. I play in rotation and if I've not been to a household in rotation for awhile they gain weight. The npcs and some pre made do as well (only if I actively played as them such as Diego Lobo, I've never played as the Goths so their weight is exactly as it was from day one)
I had my sims create clubs where one activity was to work out and they would gather at the gym to mingle and work out. Then they started going "on their own" without the gathering whenever I took the active household to the gym. They're kind of a hot spot in my game because I've used them a lot.
Instead of gaining weight they gain muscle. A lot of it. If the rotation changes to other households showing up my sims lose the muscle "bulk" and then tend to gain weight based on how long they aren't in rotation. I try to also choose the healthier meals and leave them as leftovers. So they have garden and fruit salads in the fridge. Good luck in Myshuno, the food stalls put on weight real quick.
So they're either gaining weight or gaining muscle all completely dependent on how long I'm with them, or they're an active household that just shows up at the gym because they were recently in rotation or if they fall out of rotation they gain weight.
Some gain it faster than others and some gain muscle faster. Why that is, I'm not sure. Some are very lean at the gym and others look like body builders who skipped leg day. I just roll with it and cas edit occasionally if one gets out of control.
If they're my active household I try to let hunger get to orange.