It's just the weight of their implants. No matter how small an implant you put into a girl, her breasts were only naturally meant to carry a certain weight, and thus the surrounding skin does so also. When a girl gets implants, it forces the skin to stretch, with no breast tissue to fill the void, thus the mini "craters" around the outsides of their breasts.
Yea, I wish somebody would of told me this before I got the implants.