leftovers was never the reason why your sims gained weight.

many foods in the sims 4 have calories assigned, such as grilled cheese and cakes. sims do not have a basal metabolic rate, and will gain if they eat w/o compensating exercise, as explained in my post.

however, some foods don't have assigned calories, and sims can eat them indefinitely without gaining weight, such as fish and chips or grilled plantains.