In a previous save, my sims did tend to gain weight over time, but I hated how overly muscular they looked once they started working out in gyms. In my new save, I keep them active by walking the dog and going jogging. Now I have my couple work on their wellness skills (yoga), and that seems to have slimmed them down even more. They are slimmer now than when I created them, but at least it's still a healthy look (as opposed to having an 'emaciated' look). I also have them wait until their hunger bar is yellow before they can eat a meal. Not sure if that does anything to helping them maintain a healthy weight?