Joe is right, its hard to not scale the footage a lot if you have significant shaking.
Thats what tracking tools do, they scale the footage to compensate.
In terms of the tracker in AE, from my experience its is the poorest out of a poor bunch.
Shake has a OK one and those times when I cant use AE I take it into Shake and it is always, without fail, a better tracker.
Johan Polhem
Motion Graphics
www.johanpolhem.com