Deep down, they simultaneously fear and revere the Mighty Phallus. So whatever the size of their own, they are either hugely ostentatiously impressed by it, (and other men's), or terrified that it's inadequate and the only way the can be a Real Mens is by dissing women (even though the atual clammy unappealing organ is no less than average.)
I've seen a LOT of willies. Most of them are pretty much standard size. SO, boys, get over it. It's just a willy.