I wore an ill fitting 34A/32B through school, was probably more of a 28D back then, but still itty bittys.
It was not until I turned 19 that my boobs started to grow and went up to G cups in about 3 years. Then they settled for a few more years until I finished university and then grew again slowly until I was maybe HH/J cup and then they ballooned in 2 years to my current size. I'll be turning 30 this year and they're still changing it seems, although more in shape/hang than size, unsure.
I certainly don't know what causes it, my dr also does not know, it may just be genetics/hormones/age. I was a late bloomer and even though I always wanted big boobs and I had them and I enjoyed it, then, I didn't think they'd get so big. The larger you get the bigger the cons.