People seem to forget how small (body wise) actors are, you take a skinny "pretty boy" who weighs in at 130 lbs and a petite actress who weighs in at 100 lbs and anybodys cock will look big.
Probably harder than you think. That's why few men can actually perform in a porno. Pierre Woodman (guy behind Private) once said that 1 out of 10 could actually get an erection. Maybe it's the stress, cameras, lights, people....