Well I think they appear small because the guys are involved in some intense physical activity. The roids might play a factor as would, I guess, them tucking them in good to not look too obscene. Pro Wrestling does mostly try to appeal to younger, straight men and boys.
I haven't found any pictures, but there are some guys who look bigger than average: D'angelo Dinero always comes to the ring looking packed...Chris Sabin as well (I actually bought a pair of his tights from him and there is a lot of room up front) and Cody Rhodes.
Apparently, according to Bobby Heenan, Andre the Giant was quite popular with the ladies because of his penis size... which as much as I loved Andre, I would not want to see that.
Sean Morley, whose gimmick was a porn star, didn't look huge but didn't look small either. Plus there are a lot of wrestlers who wear loose tights.
Here's a video with Shawn Michaels sported a huge bulge, though he admited later it was filled with gauze.