breast growth is a bit of a lottery and genetics play a big role... the unwritten rule is one cup size smaller than the women in your family, but this is not a hard and fast rule as for me, I technically have bigger breasts than my mother, as she wears a 34D and I wear a 36D, but then my mother and I have the smallest breasts in the family, most of the women are DD and DDD, so maybe some of those genetics played a role for me.
When I started HRT, my body reacted quickly to the hormones (like a good fertilizer) and I had quick breast growth, I was a B cup in no time (around 3 months) and I had a flat chest to start with (no man boobs), but then I was 21 when I started. After that things slowed down. I remember when my breast buds came out, my nipples were super sensitive and I had breast buds within 3 weeks. Once I was a b cup the pain cleared up and I only pain in my breasts a few days at a time after that.
I am happy with my breast size, maybe when I am older and my breasts start to sag, then I will get a boob job, just to firm things up and maybe go up a cup size.