It should be mentioned at least in passing that DD is also a bra cup size.
Seems that manufacturers started out with 4 standard sizes: A, B, C, D (going from small to big, or from Kate Moss to Geri Halliwell. But then there are always a few specimens that don't fit into this range, yet clamoring for a bra size of their own. So they gave Brigitte Nielsen size AA and Pamela Anderson (thanks to a generous implant of silicone) DD.
Size DD is eminently impractical. DD breasts keep getting in the way. They're a concussion hazard when in motion and a smothering risk when you get too close. However, I must say they're nice to look at from a respectful distance.
____________________
Reference
Breast Files, http://www.breastfiles.com/bras.html