One thing that many people are finding frustrating about The Sims 3 is that it's surprisingly difficult to change your Sim's body shape. Sims can now get fat - quite fat, actually! - but it takes a LOT of over-eating to get them there. And once they are overweight, it takes a LOT of exercise to get the pounds off.
One of those things is true to real life, and the other is not. I leave it as an exercise for the reader to decide which is which.
If you want to make your Sim fat, that's easy! Keep making them eat, even (especially!) when their Hunger bar is not empty. And don't let them exercise. The "Couch Potato" trait is a good one for this project!
You have a surprising number of exercise options, several of which are new for Sims 3. As in Sims 2, your Sim can: