From (IMO) most to least likely:

• I had an orchi way too early and completely starving my body of testosterone with the resulting high SHBG stunted growth (this would explain why my nipples got sore again when I almost detransed)

• I am simply THAT much of a bodyhon that they exist but are just spread out across my enormous chest

• I pregdosed myself for an incredible amount of time (nearly 5 years) and that fucked things up

• I took progesterone too early because I was using it as a blocker and it capped my final size smaller than a flat cis girl

• I took spiro for a year

• Genetics (Unlikely, cis and trans women in my family all have breasts)