I did do a study of overweight sims. In my study.
1. I found that what they ate had nothing to do with them gaining weight.or losing weight. So I like to give them a normal diet, with sweets sometimes, like normal.
2. I think a lack of exercise makes them auto gain weight. So if you make one of your sims traits active you have a better chance of having an always slim Sim.
3. If you do have chubby sims and you would like for them to be slim, Just have them to go for a jog 2 times a day, this can even be jogging with the family dog. In about one sims week you will have a Slim Sim, depending on how thick they start with.
4. to keep the extra pounds off just on occasion send them for a jog, that's it. they will have a slim look, but no muscles.
5. If you want to add muscles to your slim sims, they need to go to the gym or buy them workout equipment for the home.
6. Once your sim looks the way you want them to look just stop what they are doing so they do not get extra slim, or too many muscles. and go to maintenance meaning only sometimes let them work out, and keep the home workout machines in the family inventory.
7. To keep them with the normal slim look on occasion send them for a jog, like once or twice a week.
8. If you do not want your female sims to have muscles in their legs arms and body, do not send them to the gym, and do not buy them workout equipment for the home. only let them jog to stay slim.
