This is one of those things that just varies tremendously...
For me, the area under my areola started to develop first and gradually spread out. Now 2.5 years into HRT, I have A cups and very slightly hypoplastic development. Thankfully, I always wanted "puffies" ;-)
This may sound strange but you might consider piercing your nipples.... Any piercer will tell you that piercings will enlarge your nipples, especially with a larger gauge. I pierced mine (now 12g) way before HRT and they got bigger then.