Bra size is surprisingly subjective. I am lucky enough to have a girlfriend with very big boobs. The bras she wears are small on her, and the size she tells me she is, is different from the bra size she wears! I also think she is bigger than both of those sizes, and when I put her actual measurements over and under the bust in inches on a bra size calculator it told me she was even bigger than I thought! Although it has to be said different on-line calculators gave different results... And in there somewhere is her actual size...
So if someone asks me how big she is, despite knowing her boobs better than anyone, I can't give an accurate answer. Besides, a Gentleman never tells...