As the others said, the male pornstars who get to where they are, have gotten there because they are unnaturally well endowed.
I've applied for acting in a porn-movie myself, and talked to one of the lead producers and actors in Denmark (Denice Klarskov) personally. (I didn't get the role, because I was simply not big enough for what she had in mind)
What I did have however, that she found impressive for a non-pornstar, was the ability to hold back my ejaculation indefinately. (You master that technique, you'll be popular with the ladies too I might add)
All male porn-stars have some technique of holding back too, even when the female porn-star is giving her all. There are training techniques you can use to strengthen your own control over yourself.
The amount of ejaculate we see male pornstars shoot off can also be both digitally made to look larger in quantity, and whiter in texture. Failing that, good old flour+water is still a used trick in the business to make it look like buckets of ejaculate, when in reality it's most likely the same amount as what you shoot off.
As goes for enhancements.. Male pornstars DO infact sometimes use viagra.
Not 'cause their impotent, but 'cause they might need to hold an erection for a good few hours. (And unlike what you might think, viagra does NOT give you an erection, it makes it easier for you to get one and maintain it)
Hope my little revelation helped answer your question :)