small in the pants

A man with a small penis.

Fez: What are you talking about? Those men were completely average. In fact, I found the guy with the mustache downright puny.
Michael Kelso: See now, Fez, that's not honest. I mean, we all know you're small in the pants.

small pants

pants that are small enough to derive comments and taunts from others and provoke the pants wearer into untoward actions

Hey coach, maybe you should focus on how small your pants are, rather than breaking a clipboard...

small pants