This photo of Charlize Theron shows how breast tissue can migrate. Lets see the close up:
You can see that the built-in cups are way to small for her breasts. The same thing happens when we wear a band that is too big + cups that are too small: the breast tissue escapes and turns into armpit and back rolls. I have explained it here, with pictures.
If you are not sure whether your bra is doing this to you, you can do this quick test.