Cup sizes aren't fixed. The volumes of a cup actually depend greatly on the band that supports it. Butterfly Collection has written a great comprehensive description on how cups relate to bands, clearly explains the math and reason behind it, and even created a fantastic chart that makes it all easy to understand! Here's an excerpt:
"One of our most popular blog posts is ‘Not All D Cups Are The Same’ which explains that the volume of D cups (all cups in fact) gets bigger as the band size gets bigger. This is why a woman wearing a 32E can also wear a 34D because the cup volume is the same, only the band is different.
Lots of women were interested in the topic but asked for some more explanation. Today's post goes into more depth about why the volume of the same cup letter gets bigger as the band gets bigger.
What Would We Look Like If All Cup Sizes Really Were The Same Volume?As regular readers will know, cup sizes simply acknowledge the difference between a woman’s ribcage measurement and her full bust measurement. So for example both these women have D cup breasts because they both have a four inch difference between their ribcage and full bust measurements:Read the rest of this brilliant post on their website: http://blog.butterflycollection.ca/2012/09/why-do-cup-volumes-get-bigger-as-band.html
Woman A is a 30D (Ribcage = 30 inches, Full Bust = 34 inches)
Woman B is a 38D (Ribcage = 38 inches, Full Bust = 42 inches)
So why does Woman B with a 38D bust look bustier than Woman A who has a 30D bust? The answer is that Woman B's bust is PROPORTIONALLY bigger than Woman A.
To put this into context let's take the 30D breasts from Woman A and put them on women with 32, 34, 36 and 38 bodies to see what it looks like..."