Sims have a hidden weight scale and they slowly put on weight as they eat. The amount of weight they put on depends on the calorie count set on the items they eat. Some items have this value set while others don’t. Sims will gain some weight from garden salads but not from eclairs for example.
In order to make things a bit more consistent I added calorie counts to food and drink items that didn’t have any (more than 300 food and drink items). I left the ones already set by the game as they were.
To counter-balance the increased caloric input I added a daily caloric drain of about 1500 calories per sim. I wanted my sims to be able to eat multiple times a day (not only when hungry), occasionally have drinks and snacks, and maintain weight without increasing the fitness skill.
As a general guideline: lobster thermidor is 800 calories (this is vanilla), an experimental main dish is 600, cake/pies are around 400, bar drinks are 200 (vanilla), grilled steak is 300 and so are some vending machine snacks, soups and lighter items are 200, most coffees are 10 and teas are 5 calories.