Progesterone is a very big mystery. The last time I checked, there was pretty much just as much support for all claims:
-It helps breast growth, because glandular tissue is developed better under its influence.
-It has absolutely no effect on breast growth, because girls have pretty much all of their breast growth when their blood levels of progesterone are low.
-It hinders breast growth and development in general, because progesterone acts like an androgen in several ways.
And to be honest none of the points of view managed to convince me more than the others. This is mainly because no one cares about progesterone in trans women, so most of the data available is deduced, not checked.