The guys in adult entertainment undoubtedly have larger than average dicks, but, yes, most all their claims are exaggerated. Still, I think 8" is pretty normal (and 8" is a BIG dick) just based on how many times I've seen a chick wrap her hand around a cock and still have 2/3 of the cock exposed.
So, yes they have big dicks, but they also do use camera angles to exaggerate, shave down their pubic hair, and all the other little tricks to amplify the appearance.