Bigger front tires have more surface area which means more grip. As the front wheel has a greater lean angle in corners than the rear, and contributes less to overall rolling resistance, you can take advantage of a larger tire in the front.
The rear wheel's primary function is drive, so you need to be able to accellerate and decellerate it easily, and that to me means a semi-knobby race tire like the Hutchinson Python, Geax Mezcal or Schwalbe Racing Ralph.
You also have to remember two factors. One - the larger front tire is for grip, not cush. That's why you have suspension forks. Tires make poor suspension devices. Two - It's much easier to regain control of a sliding rear tire, no so easy with a front, so you can take a few concessions with a rear tire in terms of grip vs rolling resistance that you shouldn't really take with the rear.
Horses for courses though - tire selection is definitely also based on what the terrain is like you are going to be riding in, and the best source of info for that are the locals.