Morbidly A Beast:
I’ve never been quite sure how they gauge womens bust sizes.
So the Band Size (the number) is the size around your ribs below the breast.
The Cup Size (the letter) is the difference between the fullest part of the breast and the band size. It varies a little by company and country, but generally A-1", B-2", C-3" and so on. Generally the variance is after D and is in how often they use double letters/ the whole alphabet.
Because it's the difference in sizes a larger band size makes even fairly large cup sizes less impressive looking. It's how some of us are rocking large cup sizes without looking huge.