Townies and NPCs will be affected as well.

The greatest decrease-version will effectively keep all Sims right around '0' fitness and/or '0' fatness (default middle-slider body-shape), again based on what you choose. The decreased-range versions will keep all Sims in a smaller-range of fitness or fatness or both, depending on what you choose. Depending on what version you choose, your Sims will be able to achieve either hugely muscular, hugely fat, or rail thin body shapes, all through standard in-game interactions such as working out and eating.

Increases or decreases the visible range of attainable Sim body shapes, fit or bony, fat or lean. Physique Range Increaser and Decreaser - Fit, Fat, Bony, and Lean