Partly it depends on whether it's actually breast tissue or just fat; neither is good or bad overall, but as a starting point if there's genuine breast tissue there it suggests that your body really likes E, which is usually a good sign. I had gynecomastia (slight but significant breast development) pre-HRT, and I was a 42D by the end of the first year.
Monitoring growth will be easier than you think, I suspect, as growing breasts are usually both sore and "perky" in a way that fat alone isn't.

You can also use both measuring tape and a couple of "test bras" to keep tabs on things; I had one bra I liked to take out and see how much I filled it each month or so.
I also think the one cup size smaller thing is a myth; genetics play a part, but like cis girls, we get what we get. (My mother is 34B and I'm 42DDD. Heh.)