Just remember that all avaliable pieces of clothing has to fit on all possible combinations of body size and shape.
Meshs can be stretched/squeezed to fit, but only to a point (if visual quality is to be maintained). After that someone has to put manhours into adjusting it onto a each possible frame (or group thereof) everytime a new item is introduced.
That is one of the reasons there usual are restriction on how much you can change the body proportions.
Oh, and of course the same with animations.