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 solely determined by the clothes being worn, and is not affected by exercise, eating or any other activity. Body type 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. Normal is often referred to as "thin". Normal/thin is the default when creating a Sim. While the player may choose to create a Sim as fat, Sims cannot be created as fit.
The differences between the normal/thin and fit body shapes are not always visible in gameplay. Even when they are visible, they may not be noticeable unless the player looks closely. Fit Sims have better-defined abdominal muscles than normal/thin Sims, but the difference is subtle, and generally does not show through clothing.
A Sim's body shape is controlled by internal flags. These are normally set according to the Sim's fitness level, though both the flags and fitness level can be viewed and manipulated with SimPE. It is not unusual for pre-made Sims or newly generated townies to have preset fitness levels that do not match their preset body shape flags. For playable Sims, such a mismatch can be corrected by sending the Sim to a community lot or by having the Sim perform a fitness-building activity. For townies and NPCs, correcting a mismatch requires using SimPE to set the Sim's body shape flags and chemistry traits.
The Sims 3Edit
In The Sims 3: Late Night, there are new sliders which affect body shape by affecting muscle definition (for males & females) and breasts (for females).
The Sims MedievalEdit
The Sims Medieval also has body shape sliders.
The Sims 4Edit
In The Sims 4, body shape has been enhanced from previous Sims games, and can now be modified much like head shape. Fitness and fatness sliders still exist, though, and they function similarly how they did in The Sims 3. The only exception being that muscle definition is now defined according to the fitness/fatness sliders, rather than being a third, separate scale.