I was advised by a trans doctor that progesterone is very hit and miss re breast size, but that it has the function of maturing breast cells, and if started too early in trans HRT, can actually slow, or even stop development prematurely. I see some doctors prescribing P after a couple of months HRT, or even after 12 months, but the start time is best timed when development has reached Tanner stage 3. This means most of the breast size has been achieved and the P can improve shape.
I used it for 2 years, systematically trying both oral and suppository methods, and my doctor compared my blood test results for each method, finding very little difference. The only effect I noticed was an increase in nipple size. The oral method did help me go to sleep, whereas the suppository method didn't.
I stoped P because I just couldn't justify the price. In Australia, medroxyprogesterone (synthetic P) is subsidised by our health system as it reduces bleeding in cis women, but it does have extra risks trans people can avoid by using bio identical P, but I had to pay full price. I really wasn't seeing any advantages to warrant the cost.
Hugs,
Allie