In my honest opinion it's not a one size fits all kind of deal. For example there are body types that would look kind of off with enormous boobs and ones that would look off with really small boobs.
I think this may be why small petite woman don't usually have ridiculously large breasts and you don't usually see "larger" woman that are flat as a board. It seems as if nature just knows what would look great.
So I guess I'm saying that some girls look best with s boobs some look best with medium or big and some even look best flat as a board.