[Rumbl3r]

I agree, but what I'm trying to explain is why the rears should be kept 2mm bigger than the fronts as standard.

I think the reason is contact area which will change according to diameter, but we're talking a miniscule amount. If the same or similar miniscule amount is added or removed from the fronts too, maybe it keeps the balance. And as Mark's document suggests you can tune things a little by varying the balance?