Julie,
I think thats just part of the standard Tanner stages development, but for most of us who not on progesterone as well, the rounding/filling out is less pronounced.
From my own current experience, the other factor is fat deposition. During my several years of PM after all growth pains etc had ceased ( after 18 months), there was still a slow, continued, almost imperceptible, unmeasurable, rounding. I've now been off everything for 6 months and I'm shrinking, faster and faster over the past month or so, and the shape has gone back to pointy ( with that depressing saggy hollow on top). The underlying breast tissue is still there of course but the fat is going.