I can't remember where I picked up this picture, but I think it does quite a good job of challenging common misconceptions about cup sizes. Most people would look at the first girl in the set and say she has quite small breasts and may guess she was a B cup. To actually show her in the 32DD size can be mindblowing, as it was for a friend of mine when I showed her this. She currently wears a 36B and her breasts are a similar size, perhaps a tad bigger. She measured herself using the MWB method and it put her at around a 36DD/34E.
To a lot of women, DD/E cups are big, but an F is enormous in mainstream media. This is not always the case, especially in the smaller back sizes. Cup sizes are relative to band sizes, as you can see in this video from Freya.
The customer wears a 36C (Australian 14C) and by no means has an "enormous" bust, but is soon wearing a size 30FF that fits her perfectly.
JM