If you have sims near vendors, particularly in Myshuno, they're the most likely to gain weight. I have rotational play with quite a few sims in the city. When my active household is in the city the others walk through as "townies" and constantly eat vendor food.
I learned with those sims to create a club where they work out together to help minimize this. However on the flip side they can become extremely muscular because they become gym townies. Outside of Myshuno I don't seem to have the body fluctuations quite as quickly or drastically.
I've learned to endurance run most of my sims while they're the active house so when they do become gym townies they're most likely to pick the endurance run option autonomously instead of weight lifting.
I did an experiment on body types by food, location and activities. Myshuno was the most drastic in fluctuation by far. I also learned to leave salads as a leftover option before rotating homes.
The fastest way to gain weight is to have a club with the only activity set to eating. They won't stop until the club meeting ends and have them meet often for dinner parties.
For max weight and max muscle have a club set to eating and exercise only. Have dinner parties as well as gym days. This is if you want to track your other homes on rotational play.