The thing to remember is that bodybuilders only pump themselves full of testosterone during the growth phase of their training cycle. This phase finishes long before they enter competition, so when you see them on stage they certainly aren't pumped full of the stuff - in fact they usually have considerably less of it than the average guy.
I can tell you that when a bodybuilder IS pumping himself full of testosterone during the growth phase, it has a massive effect on his sexual performance. His libido goes through the roof and his penis gets massively big and hard - sometimes even causing priapism, which sounds great but actually needs medical attention. Theres no doubt this makes the penis larger - because it is stretching and filling out the blood vessels - but the change is not really permanent.
The problem is that if you take the wrong substances, the testosterone is coming straight from the needle not from your testicles. In fact, your body soon begins to ask why your testosterone levels have suddenly rocketed and in an effort to restore the balance, switches off your testicles entirely. If this goes on for long, your testicles shrink permanently through lack of exercise. This is why you sometimes see bodybuilders who have virtually no bulge in their trunks. Sad to say, these guys have almost emasculated themselves. Of course things are much better these days - at least if you get your substances from reputable sources. Bodybuilders cycle so that they arent on dangerous stuff for too long, and there are substances that get your testicles to pump up their testosterone production rather than relying on whats injected.