I don't want to be a party pooper, but often in the first few months, your breast get big fast but in fact, most of it is the muscle (with the nipple on top). I know this from experience, by month 6 I was 36C but now 1.5 year later I'm 32C which is 4 times smaller in size, why, because the muscle behind the breasts deflated completely. Also, the breast shape makes a big difference, a 36C after 4 months is unlikely to be a half sphere, its certainly conical, which means that a full cup bra will fit very badly. Better a triangle bra, a stretcheable bar, or a balcony bra.
So, in short, until we've lost our muscles, and the breasts are mature (which takes years even for teen girls), cup size doesn't really tell the real final picture.