The difference between cup sizes is one inch.
C cup is one whole cup size bigger....
C cup is larger than B cup. You can google bra sizes for more detailed information.
For your example, the a 34C is larger than a 34B. BUT that does NOT mean a C is ALWAYS larger than a B. You'd need to focus on the "cup volume" and you really need a chart for that.
cup sizes
C cup is bigger than a B cup. Check out this web site to understand more about bra sizing.
