[msnewboobies]

Basically the tissue under your arm pits and even on the side of your back is considered breast tissue and when you wear a band that is too large, the breast tissue (slowly over time) moves from the cups where it should be to areas of your back and side.
One sign of this is when you find yourself having to adjust your bra throughout the day by scooping tissue into the cups or by pulling the bra down to bring cleavage out.

With the right band size, you are supposed to bend over and lift your breasts from the sides of your chest into the cup and then the tissue should stay in the cup all day and you shouldn't have to adjust it. Check out the pic I attached.
Also, for the cup size, the side of the bra should go up almost to your armpit so it covers all of the tissue. See the second pic I attached.