Small object/vehicle issues?

[gsteinLTU]

My goal is to recreate this simple small mobile robot:
It has two powered wheels in the front, and one unpowered caster in the back, no suspension, and only differential steering. After a lot of trial and error, I have a model of it that works almost perfectly, except it only functions as expected when it is scaled up significantly (~3x normal size or more). At the "real-world" scale, it is capable of going forwards and backwards, but turning does not work. Otherwise, it's working a lot better than the previous physics engine we were using!

Is there any advice on tweaking things to make this sort of small "vehicle" work better? I can scale the rest of the simulation up to match, but it would be a lot nicer to not have to do that.