I've noticed that women tend to allow their shoulders to be rounder than men. I can't say this always accounts for their shoulders appearing to be smaller, of course. But if you allow your shoulders to hunch a little, in the way the PE teacher, at school, alwasy told you to correct, you will notice that the width of your upper bidy is slightly reduced. It's also rather relaxing.
As you develop breasts, I would imagine this will become automatic. Perhaps a reason to wish for big boobs?