sliders or just adding some smaller characters?
The game isn't an actual 3D enviorement, its more like screenshots and renders put together to make a scene, and so to make what you are suggesting, the dev would have to redo the entire game with every single size combination possible, which would be actually impossible, and even if there was just a single option of having the characters have anatomically correct proportions, it would still mean the dev having to redo every scene in the entire game.