You need fat on back because heavy load is there. Otherwise both would be skinny. Situation is different nowadays when there are also easy rolling fat tires. I

You need fat on back because heavy load is there. Otherwise both would be skinny. Situation is different nowadays when there are also easy rolling fat tires. In olden times 47-622 was always just very knoppy. I soloed the Alaska Hiway with this arangement in 1987. In olden times you also had to be very careful with the back axle. Until Shimano improved the design.