Behold the elder-lore, once sung 'round internet campfire tales of yore! /j

Transfeminine Science has a thorough discussion of the concerns:

These suggestions include progestogens having known antiestrogenic effects in the breasts, animal studies finding stunted mammary development with high doses of progestogens, clinical publications cautioning against premature introduction of progestogens in female puberty induction due to concerns about possibly stunted breast growth, clinical use of progestogens to treat macromastia in cisgender females, poor breast development with estrogen therapy in cisgender girls with a disorder of sexual development that results in high progesterone exposure, and breast development with estrogen and CPA (a very strong progestogen) typically being poor in transfeminine people.

Folks very much are starting earlier now! A recent paper noted that, “[m]ost patients who started progesterone began after 1-6 months on a standard GAHT regimen (59.3%).” But only two women were doing rectal administration in that study, which is what everyone online says works best.

My first year of hrt was all diy. I added progesterone after a bit more than eighteen months of slowly increasing estrogen, with the advice of a doctor, around seven years ago. At the time my dose of e was 6mg/day buccal, and p was 100mg/day oral for the first six months, then the other way. Maybe a year after that, a different doctor increased p to 200mg and eventually started me on injections around four and a half years ago. It’s been more than eight and half years now since I started hrt. The younger woman I was when beginning would be thrilled with where the girls have ended up, but I’d still like them a bit bigger.