I'm no expert but I believe most MCs have chunky rubber on the back for lots of grip to move you forward and to throw off any mud you might ride through.
The front is usually slightly narrower to allow for better steering control and water displacement. You don't want a fat sucker tire in the front or your steering might feel sluggish.
In the back, provided you have sufficient clearance, a fatter tire can give you a little more cushioning. Check your clearance to see if you can add a slightly fatter rear tire. There is a recent chopper trend to put a bad ass fat tire on the rear and a skinny thing in front.