Melissa, progesterone helping with breast growth is not just anecdotal, there are studies that exist to support it. I'll have to go into the past message archive of the DIY to find them. More and more endo's linked to gender programs are precribing it.
It also has an impact on the skin, nails, hair, ligaments, energy level, libido, appetite, fat distribution and lipid profile (and probably other things I'm forgetting about). So... Not too bad for a useless (sic) hormone.
Since progesterone is needed for the breast to fully devellop its milk making apparatus and is a major reason for increased appetite and fat storage in gynoid regions (the breast being one of them) during pregnancy, it goes to reason that it has a significant impact on the breast!!!