Make sure your sim isn't eating a lot of certain foods as they do have calories to them. Grand meals that came with seasons get your sims fat in no time. Also, another thing if you happen to do rotational and play in city living sims are 'constantly' going to food stalls and eating all the time, so they also end up gaining a lot of weight there too.
Tend to have my sims jog or be on a tredmill they won't gain any muscle definition from it. :)
I have a vegetarian sim and she always maintains her weight as she's eating salads and raw veggies. Drinks from the bar I think is like 200 calories a piece? Drinks from the fridge is 0.
A few food items that have rather low to average calories to them is:
Ambrosia, 0
Cereal, 120
Chips, 200
Fish Tacos, 0
Asparagus proscuitto wrapped, 100
blackened bass, 0
chips, 200
garden salad, 50
fish and chips, 0
grilled chicken, salmon, or steak, 0
oatmeal, 166
pan fried tilapia, 0
fruit salad, 70
french toast, 200
Hamburger, 350
Hot Dog, 350
Mac N Cheese, 200
Pancake, 200
Parfait, 290
Pasta Primavera, 251
Scrambled Egg, 155
Spaghetti, 300
Yogurt, 79
Word to the wise pizza has some high calories like 500 and lobster thermidor is like 800 at least? Had a sim who just lived off pizza. He got pretty fat eating that three times a days.