Well can they? I don't remember them ever doing that before but a couple times I've played a Sim, I've noticed their body looks different after a couple days or so. That is happening with my current S...
In the legacy im playing now, my female sims are all overweight. They inherit the same big waist... I had one of them exercise a lot and eat salads and she lost some weight but then i made a cake for a birthday party, she ate two pieces and she recovered some of those lost kg...