The problem is that, in porn, there's people who want to see big boobs and people wo want to see small boobs.
People who want to see small boobs want A or B cup.
People who want small boobs want DD cup or even bigger but they are not interested into C or D cup 'cause since now many girls have DD or bigger, if you want big boobs, why would you settel for C or D when you can have DD ?