Rakel, the cup size totally depends on the band size. The B cup in a 46B is much larger than the D cup in a 34D.
There's a chart on this page that illustrates this well. WARNING - not work friendly - pictures of the, ah, subject material (plastic surgery site). It determines cup size by a combination of ribcage measurement and actual breast measurement (from side of breast, across to center chest).
OOPS, didn't actually post the link! Here you go:
size chart