Kristyn : That idea is going around a lot too, but from what I've seen it's not true. In fact, I don't know of a single trans woman whose breasts were fully mature within 2 years. It's true that we don't have the gradual ramp-up cis girls do and their bodies usually do take a couple years to get up to "full speed ahead," but even so they can take anywhere from 5-10 years from beginning genuine development to fully mature.
I'm 5 years on HRT and just now noticed my bras are fitting tighter again... and I developed really fast early on, so I'd actually wondered if I might be done in 2 years even if most women aren't.
(Oh, and I'm several cup sizes bigger than my [very thin] mother and at least 2 cup sizes bigger than my [curvier but small build] sister, but I also had a busty grandmother. Genetics are weird.)