In my game it's just the female sims that seem to get fat. I start with married couples, both of them at a regular weight, but soon enough the female balloons up. However very few of my sims actually go to the gym, some I send jogging around the neighbourhood, some will do exercises in their livingroom, but most of them sooner or later will visit the restaurant where they order Diet icecream. I also sell it in a shop, they buy a bucket and stick it in their fridge and I let them eat icecream when they are hungry.