Well, it actually does. It’s what’s called sister sizes. Depending on the cut of the bra you may be able to go down a band size and up one cup size, or go up a band size and down a cup size and have it fit. The main issue is how the band fits, even if the cups have the right volume. A 34dd, 36d, and 38c will all have the same cup volume, but if the band is too tight or too lose the fit will be compromised.
I’m told all the time that my boobs do not “look like” the size they actually are. I’ve always said a gentleman is free to come out bra shopping with me sometime or if he would like to see for himself how difficult it is to find a bra that fits me. I have never had a large band size, not even when I was hugely pregnant.
The volume of breast indicated by the cup size is different as relates to the band size. They measure around your rib cage under the breast first and, to get your band size. Then they measure around the fullest part of your bust. The cup size is determined by the difference. 1 inch bigger is an A cup, 2 inches bigger is a B cup, 3 inches bigger is a C cup, 4 inches bigger is a D, 5” bigger is a DD, and so on. So a slender woman with DD’s will have breasts that do look significantly smaller than a large woman with DD’s.
My 34G breasts have much less volume than a 42G. They’re big enough to be a huge pain in the ass for bra shopping (certain VS bras in a 36DDD fit ok) but they don’t look ridiculously huge, which kinda sucks. If they’re gonna be big they could at least LOOK big. Lol.
And as stated above, most women in America are wearing the wrong size bra and have no idea. Most need a smaller band and bigger cups.