Going back to mountain bikes- 15 to 20 years ago it was normal to run different size tyres. But the larger tyre was on the front. Supposedly this was to give better steering. That may work in the "Dry" winters of the US (There must be some bits that don't have too much winter rain)- but over here wide tyres do not work in mud. They skate over the surface of the gloop offering no grip and if I was going to lose traction, I always felt it was better to lose the rear tyre.