In The Sims, body shapes are generally referred to as body types. The Sims has different clothes for every body type. The basic body types for adults are fat, fit and skinny, though custom body types are possible. Children have their own body type, and can also have custom body types.
Body type is determined by the clothes being worn, and is set when the Sim's initial outfit is chosen in Create a Sim. Children who become adults by using the Age of Instant charm[TS:MM] get a random body type. Prior to Hot Date, using a dresser could change a Sim's body type. Even after Hot Date, custom dressers based on pre-Hot Date dressers could still do this. Some third-party character editing programs may allow a Sim's body type to be changed.
The Sims 2Edit
In The Sims 2, Sims can have three body shapes - fit, normal and fat. They cannot be fit at creation, though their fitness level and body shape flags can be manipulated with SimPE. It is affected by eating or working out.
The Sims 3Edit
In The Sims 3, obesity and muscularity can both be varied on a continuous scale. They are affected by eating and exercise. In The Sims 3: Late Night there are new body shapes sliders: muscle definition (for males & females), breasts (females).
The Sims MedievalEdit
The Sims Medieval also has body shape sliders.