They bug out when he really wants to express things. Like if I'm eating a snack in the den. He'll just sit there very quietly, and if I look down at him, his eyes get about twice as big.
I've done a bit of voice over work, so I started doing voices for the dog a while back. I picked a Peter Lorre voice for the dog, because of those bug eyes.
"Geeeve eeet to me show (Joe). Eye waaant dee snaaaaack!".
Here's a great picture that Erin took of him for her website. They were playing "pull the sock". He's so small that when you give him any resistance at all his feet skate along the floor of the den trying to gain traction: