In the 'Quarters' area you'll find a computer (the thing with the face). Press the interact key while in front of it to open the computer menu. Weight gain controls are under 'Diet Control.'
The topmost option (Regulated/Unregulated) disable or enable weight gain.
Regulated = Disabled
Unregulated = Enabled
You can use the other settings in the menu to change how fast or slow you gain weight
Hope that helps