It is not a secret that a lot of men like watching women with big breasts in porn. Big breasts are something that is considered a cultural value in the majority of modern cultures. Men everywhere tend to prefer women with bigger boobs than with smaller if talking only about physical parameters (when they like a particular woman, the size of breasts really does not matter that much).
Some scientists believe that this preference has its roots from the ancient time when it was believed that woman with curves will have an easier time carrying many children, and obviously a woman with big breasts will have easier times feeding them. So such women were valued for propagation, and as a result, the image of a woman with big breasts as more attractive was rooted in the unconscious of most men.