I was on a reasonably low dose and they finally stopped growing after more than 5 years at perhaps a large B. I figured that was about it, but I recently started on a full transitioning regime, and they have not stopped hurting or growing since. I was originally on estrogen only with blood levels of around 200 pmol/L, which was clearly not enough.
For me at least lower dose does mean less breast growth, even over many years. I don't think the actual dose means much though, its the blood levels that are important for effect and safety. I was using patches originally, and I'm told that some people don't absorb estrogen very well from them.
I'm not sure what would happen if you had too much estrogen, but I'd guess it wouldn't limit breast growth, unless it killed you first.
If they keep going for another 5 years like they are now I think I'm going to be in trouble.