I've just started HRT, so I can't really comment on my personal breast growth. I can say my nipples hurt. They are towards the bottom of my breasts but they point straight out, not downwards. I seem to be developing a lot of upper tissue between the nipple and the armpit, but it could have always been there and I'm just noticing it now. I don't know.
I am curious though as to why MTF's never really fully develope breasts. Anyone have an answer to that? Or am I wrong, do MTF develope full breasts? If I had to guess, it would be because as the body ages the estrogen receptors in the breasts and other bodily tissues finally give up on waiting to receive estrogen and so when estrogen is finally introduced--whether it be in the person's 20's, 30's, 40's, 50's, 60's, 70's, 80's, 90's (I didn't want to exclude anyone, so I'm guessing that no one begins HRT in their 100's, but if you have, I'm sorry!)--the estrogen receptors are just no longer there and active.
On a side note, but still breasts related, I had a crazy dream: I woke up and was naked for some reason. But I looked in the mirror and I had fully developed breasts! I started feeling them. And then I was trying to decide who I should call first and tell. I couldn't decide though, and the conflict caused by my indecisiveness caused me to awaken.