In those pictures, the big one doesn't really look that bad. Sure they're medically overweight but not drastically so. They still have curves and no rolls or anything. They don't look gross at all. Whereas the skinny ones are very skinny and don't look remotely good at all. I feel that if the thin ones can be as thin as that than there should have been more options with the big ones. We should've been able to make them bigger.
This is the sims afterall, not real life.
Note: I don't mean to body shame any body type, I'm talking about the sims not real people, and I apologize if my comment hurts anyone.