In the frustrating world we live in- it depends.
Listen- cup size depends on band size.
See these bras? The cups all look the same right?
That's because they are sister sizes.
The cup volume is the same on all 5, but the band is different.
I'm holding the hook ends flush and as you can see, the eye ends are all different lengths. In fact, the 38B is about 8" longer than the 30DDD.
So when someone says "I'm a C cup" that means nothing without knowing the band.
What that also means is a DD may or may not be that much breast tissue. A DD on a 32 band is going to be the same volume of breast as a B cup on a 38.
Most department stores carry limited sizes and they use this sister sizing idea to limit the amount of inventory they carry. Instead of having the 30DDD, they'll just fit you in the 34D and call it good. Problem is it isn't good. The 34 band will be too loose for that 30" around person. It won't be supportive. It'll ride up, shift around and be generally irritating.
So find your locally owned lingerie shop and go get a fitting. Let them help you find out what sizes and shapes work best for you. Try new styles, try new sizes. Look, if you don't like the bras you have now, why would you keep buying the same thing?
We're taking walk ins and appointments- book here. We're here to help you feel amazing.