The title of this thread shook me a little. I mean, thinking of all the trouble simmers have with especially female Sims gaining weight just looking at a serving of garden salad.
@wylmite1986 Sims could also gain and lose weight in Sims 3. In Sims 4, though, it's gone a little overboard and you really have to watch what your Sims eat and DRINK. Bar drinks contain tons of calories and will make your Sim gain weight rapidly. So, watch it if you have a bar in your Sim's home and they have the mixology skill. If they also are a Foodie, then put the bar behind a locked door or get rid of it altogether.