Cups and Measures shared a clear and informative description she found on reddit in her blog.
"The letter is a measurement of the difference between the whole boob measurement and the around the ribcage under the boob measurement. So as the number (ribcage measurement) gets smaller, the same sized “boob measurement” gets larger, as the difference would be growing, giving you a higher cup size.So in this case, she thought she was 34C and found out that she was actually 32E, right? Her “boob measurement” (presumably 37″) is the exact same it was, meaning her boobs are actually no different, but the “ribcage measurement” is two inches less than she thought it was (she thought it was 34, but it was actually 32). This means that the difference between the two is actually 5″ different instead of 3″ which, according to the chart given to us by mywifesbusty, changes her assumed cup size from C to her actual cup size of E.The easiest way I can think of this is imagine huge boobs on a small woman. Then imagine the exact same boobs on a larger woman. The boobs seem to be more noticeably large on the little woman, giving her a bigger cup size out of the deal."