Yeah, I wouldn't say it's a bug either. They do live their own lives and if you play certain aspects a lot (like restaurants that have the tendency to have the same people there all the time), they get fat.
I have that a lot when I play rotations at apartments, because it's always the same townies every day visiting the food stalls and all they do there is eat, eat, eat 🙂
I had Anaya Jang turn into a super fatty, so when she was next to her husband Bako it looked kinda comical. For her, I had my scientist sims make the slimify potion and offer it to her. She is nice and thin again.
For a more gradual weight loss, I noticed one of the townies my sim is friends with was putting on weight. Invite them to the gym a couple of times. Once they use the equipment and gain some fitness levels, they started become gym regulars and would appear there on their own while I was playing other families rotating around at the gym. They soon become slim. And muscular if there's weight machines.
Sometimes I noticed that townies that were getting fat and I invited them to the gym became joggers and I see them around now losing weight on their own.
It really is a fascinating phenomenon trying to manage the weight of townies you don't play. I'd say it's a feature rather than a bug. I like how they can change, rather than stay the same all the time like when I played The Sims 2.
