It’s situational for me, heavily dependent on sims traits, personality, and my story for them. I have some sims who seem to naturally stay slim, others who have gained a few virtual pounds, especially females after a couple of kids, and some who have lost weight through diet an exercise, and others who exercise to varying degrees of obsessive. Right now I am playing a family whose patriarch is working his away up the political ladder and his wife is yoga obsessed. She’s not bulky but even after 4 kids she’s sporting an admirable 6 pack.