It started with my own Sim. She started gaining weight gradually over a period of time. I thought I was just imagining it at first, but then I noticed she had a lot of extra mass around her stomach and thighs. I tried to get her to do sit ups on a regular basis but she wasn't feeling it. So I ended up just using the potion you can earn as a reward for 750 pts. That made her weight go back to what it was. Since then I've been monitoring her food intake and trying to jog once a week. She has kept the weight off..
I can't say the same for her friends. All of her friends are gaining weight all around her. (Except for Bella Goth of course. ) I walked through the park and I saw that Sims that used to be thin are now overweight. I'm wondering if anyone else is experiencing this in their game?