Wow, I thought this thread looked familiar.
I keep my sims who don't exercise thin by either having them eat at least one low to zero calorie meal a day. Garden salads, fish tacos, and for quick meals, yogurt, are pretty safe. Then they can enjoy something else later. If you get the hardly hungry reward, they get hungry far less often and can often get by on one satisfying meal per day, and then it doesn't matter what the calories are, really, as long as you stop them from autonomously grabbing milk and juice from the fridge, or snacking on leftover cake if you leave them idle.
Lately I've noticed my sims looking a bit too thin. When I took them to a pool, I noticed that I can see the outline of their ribs and spines. But that might be because they have the lazy trait, so I never make them work out to gain muscle because it gives them a bad moodlet.
