Hey there! I have wondered about this too and what I've discovered is that it really all depends on the woman and what she is doing to lose the weight.
While I am fairly large breasted, when I diet and walk my cup size does not change much at all but I am losing back fat so the straps on my bra fit looser. I have a friend who was average-stacked and all she has done is diet. She lost her cup size very quickly but not much anywhere else.
I asked most of my female friends and acquaintances and the results were the same.
So it really all depends on you. Do chest exercises to help maintain support and definition for your breasts, work the back and shoulders carefully but thoroughly along with some belly busters. Good Luck!
