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 2
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 3
With an update introduced with The Sims 3: Late Night, there are new sliders which affect body shape by affecting muscle definition (for males and females) and breast size (for females).
The Sims Medieval
The Sims Medieval also has body shape sliders. They are identical to The Sims 3 before the update that introduced the sliders for muscle definition and breast size. Unlike in The Sims 3, in Create a Sim, the sliders are located on a page that also has sliders for age modification and a single face detail slider, along with the three traits a Sim can have.
The Sims 4
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. They function much as they did in The Sims 3, except that muscle definition is not a separate, controllable scale, and is instead modified by changing fatness and fitness, with higher fitness increasing definition, and higher fatness reducing it.