[GorgeousBlonde]

I found the study confirming what Isabelle said:

"Progesterone signaling through progesterone receptor (PR) is essential for lobulo-alveolar development that accompanies pregnancy, but not for ductal growth accompanying puberty."

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC16189/

So estrogen equals visible growth, while progesterone equals roundness and how much milk a breast can produce and store.