[AbiDrew85]

My understanding of growth is that estrogen starts the growth but for the body to complete the growth/ move through the stages to fully developed breast cells you then need progesterone.
So maybe you aren't building a tolerance, but your body has done what it can with just PM and you now need progesterone to complete the process and then start that cycle again, estrogen then progesterone.

Pretty much... They're both required and if anything the progesterone stage has more to do with their end size than estrogen.