Let's go on the conservative side and say that the average women is a 36C. However, since she's probably following the add 4" rule to her underbust then she's actually a 32 band size. That means she'd also need to go up two cup sizes (because she has to go down two band sizes) so that would actually make the average bra size 32DD! However, if a woman is wearing a band size that's two sizes too big then it's hard to see if the cups actually fit correctly so that means her cup size might actually be even bigger!
Does this make sense? Well, according this the average measurements according to race are:
If we assume that the waist=underbust measurement (a fairly safe assumption within an inch or two) then we can find the average Freya size (from Biggerbras.com) for US women according to ethnicity
- White: 41-34-43Black: 43-37-46
- Hispanic: 42.5-36-44
- Asian: 41-35-43
So umm... regardless of race EVERYONE is on average a DD cup or higher. Clearly there are a whole lot more D+ cups out there then people assume.
- White: 34F
- Black: 36F or 38DD
- Hispanic: 36E/F
- Asian: 34F or 36DD
Now, let's investigate this further. As I've discussed before all D cups are not created equal so a woman with a 28DD cup and a 36DD are going to look quite different. To see this just check out this blog and the picture there from Balkonetka
Actually, G cups don't even look massively huge if you ask me..
What I suspect is happening is that all these women wearing 36C and 36D cups are in such poorly fitting bras that they really don't know how well their breasts can look in well-fitting bras.
Now, another problem is clothing manufacturers. Clothes are typically made to fit a B cup, well, that just makes no sense at all! Here's an interesting discussion on the way clothing fits. Here you seem some reasons why fit is so poor now-a-days and here you can see why manufacturers are not adding more plus-size (even though the average US women is a size 14, so right on the cusp of plus-sized).
So why is the fit of clothing such a big deal? Well, I'll have to put up some more outfit posts once my new bras arrive but basically it boils down to this. If you're wearing an ill-fitting bra+clothes that are built for a B cup even though you're a lot larger then your boobs are going to sag, you'll probably end up buying a shirt with a waist that is too large and it's really hard to SEE that you have a large chest. Instead you just look more overweight. A well-fitting bra + well-fitting clothes allows others to actually see your figure and it's more obvious that you have large breast. So many of these "36C" women are actually making their busts look smaller and their waists look bigger with their current choices in clothes/bras, probably the opposite of what they want!
EDIT: I wanted to add a link to this blog that discusses the US denialism of larger cup sizes.