In my case they did grow somewhat. The nipples are thicker and stand out more than before. The areolae broadened from about 3/4 inch to about an inch and a quarter. The areolae are somewhat darker than they were before HRT. Before HRT and transition, I didn't have much sensation in my nipples. They are much more sensitive now. There's a lot of erotic sensitivity that's very pleasurable. Of course, it's also more painful if I accidentally bang into something with my boob.
Since beginning HRT, my breasts overall have developed to a good A cup. My doc speculates that there may be some additional development over time, but not a lot. Some ladies might want breast augmentation at this point, but I'm happy with the way they are right now. They have a really nice shape and they feel great. (Guys like them, too!) In any case, I'm built kind of slight/slim, so 34A looks right on me. If I got them built up to, say, C cups, I think they would look out of place on my frame. Also, I love having the option to go bra-less if I want to. If I'm going out in public without a bra, I make sure to wear something with good coverage, because my nipples would definitely stand out if I wore something sheer or translucent.
All of the changes I have described were very gradual. Don't expect sudden changes; that's not how hormones work.
Overall, I'm really pleased with HRT's effects on my breasts. It also changed the feel and tone of my skin and my hair in ways that I like. I'm very glad I went forward with HRT. It's working for me!