Body shape
Sims can have different body shapes. Pregnancy also affects body shape.
The Sims
The Sims has different clothes for every body shape. There's fat, fit and skinny, though custom body shapes are possible. Children have their own body shape.
The Sims 2
In The Sims 2, Sims can have three body shapes - fit, normal and fat. They could not be fit at creation. It is affected by eating or working out.
The Sims 3
In The Sims 3, obesity and muscularity can both be varied on a continuous scale.