To be honest I have never really been a 'tit' man but I have seen my share. I just thought all the big ones were like D or DD..
Mine are 36D, and honestly I don't think they look very big to me. Other say they do, but I don't see it.
The cup size on a bras is proportional to band size though. The band is the measurement around the chest below the breast. So a D cup on a 38" band bra is bigger than a D cup on a 36" band bra.
So, 36D is a smaller breast than a 38D.
All D's are all pretty descent sized, but not really huge compared to what Molly was talking about.