What is the difference between A vs B and C vs D bra?

The difference between A vs B and C vs D bras, assuming the band size is the same for each cup size, is one inch.

In cup and band bra sizing, the letter represents the difference in size between the bust and band. A = one inch, B = two inches, C = three inches, D = four inches, and so on. Therefore, the difference between each cup size is one inch of cup volume.

That said, perhaps confusingly, there's very little difference between a 34A and a 32B. That's because cup size is proportional, so when you change the band size, you also change the cup size. This is called sister sizing. When comparing cup sizes, make sure the band is the same, otherwise you won't have an accurate comparison.