I can't remember exactly, it was a good while ago...
but from what I can remember, the diff gear was smaller and the diff sat too low down, and I seem to remember the outdrives being either longer or shorter than the original XXX box (and driveshafts to match).
all of which lead to a car that only even wanted to go straight when you got on the power. it often didn't matter if you were steering left or right when on-power as it would go straight anyway
the cynic in me will also say that when Losi were searching for a 'new' car the development of it tested the old XXX box with some XX bits and it went OK so there is a massive saving on tooling right there.