Bra size is something no one seems to really understand properly and its a real bug of mine.
In my teens I never knew 28 or 30s existed and it was thoroughly depressing as nothing fitted and everything was uncomfortable. I was 24 before I was measured by a lady at Selfridges at Trafford Centre in desparation and it was a revelation. Back then there were few brands that did sizes, but they are becoming so much more mainstream as retailers actually start to wake up to how many women have been wearing the wrong size for years.
The cup size is a differential measurement, not a size as such. So someone wearing a DD who is a 36DD will have bigger boobs than a 32DD.
I think a 32DD has the same cup as a 30E, 34D or a 36C. (Go up one back size, go down one cup). A 32C is the same as a 30D, 34B or a 36A etc.
In proportion to your body a 32DD is big, in comparison to other women it is not. If that makes size. Which is why a disagree with what Letchlady says. It doesn't work like that. A 32DD is not average for women with that size frame - its big.
M&S are terrible for fitting. Beyond terrible. My rule of thumb is if you are normally a dress size 6/8 or below, and they don't stock a 30 back don't shop there, as they have no clue about fitting properly.
I'm a size 30E (sometimes F) and the number of people who have said "thats not even a real size" is surprising. It ignores me.
If you think its bad here, try the states or europe. We have the best bra retailers and manufacturers in the world. Bar none. When even celebs like Katie Perry are wearing the wrong size there's something wrong. (the back of her bra rides up which is a sign she is wearing a back size which is too big - she claims she is a 32 back. She is, without doubt, given her other measurements a 30).