I doubt whether porn stars feel they're "not enough." Why else would they be porn stars? And if they're "not enough", they knew what they were in for.
For actors appearing naked in non-porn films, I'd suppose that varies from actor to actor. Colin Ferrell apparently can't keep his pants on in front of a camera, while Mark Wahlberg used a cock double in "Boogie Nights". Peter Sarsgaard's flaccid penis was noticeably small in "Kinsey". He once commented that he was sure he'd make a fool of himself, and in an interview with Conan O'Brien admitted he was totally terrified when he thought he and Liam Neeson, who's notoriously well-hung, would both be nude in the scene.
Of course, there are many smaller-hung guys who'd be turned on by appearing naked in a film. It's a fantasy of mine. Wow! The rush at a restaurant when the waitress says she saw me in the movie.