I love making sims fat for some reason, but I keep my legacy people and nice families beautiful.
Ice cream makes sims fat too, you know.
Oh and also, when I was testing how to make sims fat, I noticed that one second they're eating ice cream, and the next second they're done and they are magically fat.
Literally. Even the portrait changes (for all that chin flab-ugh!) with a snap of fingers. Creepy! In sims 2 they would hiccup.
Diets do not lower weight, but they don't gain any more weight. (Diet commercials are a lie! You don't lose weight, you just gain anymore. Also, being able to still eat dessert from those diet programs makes no sense.) Make sure they still exercise.