The Compass does tend to have a very poor fit on some kids. It seems the lapbelt path isn't far enough forward to position it properly.
The Recaro Vivo is a great fit on smaller/skinnier kiddos. Another good option is the Graco Turbo Booster.
I have both and they both fit my DD wonderfully. I do prefer the Vivo because it's a bit heavier and sturdier than the Turbo, and there may even still be some sales going on right now. (I paid about $100 for my first Vivo, but got a second one for about $60 not too long ago.)
Here's some pics of my DD in the Vivo, and I've seen other users' photos that seem to fit just as nicely.