I'm going to tell you something that may blow your mind:
DDs can be small and B cups can be big. In fact, some DDs are smaller than some Bs.
That's because there's two parts to bra sizes: the band size and the cup size. You knew that though. What you may or may not know is that cup size is *relevant* to band size. The bigger the band size the bigger the cup and vice versa!
Let's start with the basics.
To find your bra size you measure directly under your breasts. You are basically measuring the circumference of your ribcage right beneath your boobs, where the band of the bra sits.
To measure the bust you measure around the fullest part of your breasts.
Here's where it gets tricky.
Some fitters add 5" to the underbust measurement and round to the nearest even number. Some don't add anything.
Either way, you subtract the band size measurement from the cup size measurement.
A one inch difference = A
2" = B
3" = C
And so forth and so forth and yes, it goes on forever.
According to the first method I'm a 32A. According to the second I'm a 28D.
I wear a 32B, which fits just fine in the band and the cups are slightly snug but a 32C would be a bit too big.
My boobs are awfully small but yes, I'm technically a D cup according to some fitters.
Remember when I said cup size is relevant? Allow me to demonstrate.
A 32DD holds the same cup volume as a 34D, and a 36C, and a 38B, and a 40A.
A 36AA holds the same as a 34A, and a 32B.
Every time you go up a band size you go down a cup size and vice versa.
So in these discussions of "what's the best cup size?" You all aren't talking about the same thing. If you think C cup is ideal, well, a 28C is the same cup size as a 30B , 32A, and a 34AA. A 36C has the same cup size as a 28F.....
Bottom line: you can't discuss cup size without band size.