Im not sure of my breast size? Help?

Well I think I am a 34 B , all 34 B bras suit me good. But from my measurements we see I should wear an A cup, but it is not so comfortable for me. I saw breast pics of girls on the internet and some 34 B cups seemed smaller then 34 A cups. How is it possible? And also do you think I should wear an A cup?
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Anyone?
Im not sure of my breast size? Help?
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