The only reason to have smaller rims in the front than in the back on a FWD car woould be to make the front wheels lighter so that more power could be put to the ground. But if you did have larger rims in the rear, you would have to make sure that the overall diameter of the front and the rear tires matched up, or else it would be pointless.

You may see some RWD cars with larger tires in the rear than in the front, but that is because 1) they can't put super large tires/rims in the front and still handle/turn well and 2) you need larger tires in the rear to help put down the power.