Everyone knows D cups and above are for large breasts, and A cups are for small breasts, right? Actually, that's not the whole story. Sometimes that's completely false.
Short answer: The cup size without the band size is not enough information. Not all D cups look the same! A 34D and a 42D are totally different sizes.
What are cup sizes?
"Honestly, I have no clue how they get the C, D, etc." —Lesly, 45
The cup size is the difference between your full-bust measurement and your band size.* Typically, each inch of difference represents one cup size increment. An A cup is 1" larger than the band, B cup is 2" larger than the band, and so on.
*There are different ways to calculate band size. The traditional method is to add 4" to the underbust measurement. Adding anywhere from 0" to 4" may help you get the best fit and support. The stretchiness of the fabric and your breast shape play a role.
Within the same band size, cup size increases do mean larger breasts. 34A is smaller than 34B, which is smaller than 34C, and so on. But within the same cup size family, the larger the band, the larger the volume. The cup size is relative to the band size.
Sometimes you can find a better fitting bra by trying on a sister size. Sister sizes have the same cup volume but on different band sizes, as shown in the image below.
The woman who's a size 30F may get fitted correctly for the first time and think, "F cups are huge! My boobs aren't that big. 30F can't be right." She feels cup size sticker shock. It's a common reaction. Misperceptions about bra sizes are everywhere.
Cup sizes do not end at DD/E even though many brands stop there. G cups are common. There are even cup sizes beyond K.
Remember: you're a person, not a cup size. Wear bras that fit you well and feel comfortable.