All Head, No Shaft

Analogous to "All Bark, No Bite". A person who is a loud, obnoxious, overly aggressive jackass, but without anything to back up the behavior. It's is like having a thick two-inch dick with a huge head, impressive to the owner but basically useless.

"Look at this feeble dork acting hard and trying to boss everyone around;

He's All Head, No Shaft."