mirona

This was done way before I started BMX, so it's and old trick. Since there is more weight on the rear tire, you only need a skinny tire to get enough traction without causing undue friction that you would have with a wider tire (rolls easier). The wider tire is put on the front because it will put more tread on the ground and effectively give you better handling through the loose stuff.

:edit: The reason you have a wider tire on motorcycles is because there is way more power going to the rear wheel of a motorcycle than a bicycle, so you need more area to grip whatever surface you are on.