Stand your ground, man!
One more time. Cup size is defined by the # of inches in the difference between band size underneath the breast and bust size at its fullest. One inch per cup. It's in absolute difference in inches. It's not a percentage difference. 1 inch difference = A cup, 2 inch = B cup, 3=C, 4=D, etc.
Thus, a teeny 4'10 girl with a 2-inch difference is a B cup. But scale her up to 5'5, she'd have closer to a 3-inch difference and be a C cup. (The math doesn't quite literally work in those examples. But, you know, rounding. 2.2 to 2.8, or something. Work with me).
So, literally B, but visually/emotionally C. Both sides are right. Very fine people on both sides. Aya's rack is also so right and very fine, regardless of the label.