I don't think its respect exactly. I do think there is some change in attitude, though, it seems to be more of a dominance dynamic. I think we treat large penises as being more masculine, but its not limited to penises. If you're standing next to your friend and you both take your shirts off and you've got a spare tire while he's got a six pack, you're probably going to feel something similar to if you both dropped your pants and he had a bigger penis. Or if your friend can somehow display that they are more intelligent than you, you might feel the same way.
I had a coworker once who was a real asshole to me all the time, he always had a shitty comment for everything I said near him. At some point he and I started going to the same gym, but obviously not going together. After working out one day I stepped out of the shower naked and he was walking into the shower area also naked. His penis was quite small and mine was much larger than his, and he obviously noticed. He never got friendly after that, but his comments stopped completely after that experience.
I don't think that guy started respecting me after that. I think he felt more like no matter what he could say to me I would always win because I had a bigger penis. I think the same thing could be true if I had a much better body than him, or was obviously much more intelligent than him. While he might respect me, he will almost certainly have some feeling of inferiority or submissiveness.