It's all about surface area.

2" wide belts seems to be the preferred size for many knife makers, but I think it has to do with the availablilty of 2 X 72" belts for machines like the KMG.

Most 30" and 42" belt machines I've seen run in 1" wide. The wider and longer the belts, the more surface area and cooler it runs, so belts last longer.

I've grinded knives on both a KMG (2x72") and my delta (1x42"), and the KMG just tears the delta apart. First of all the KMG had something like a 3HP motor vs my 1/3 HP motor. Secondly when profiling knives, you take the knife to the belt at a perpendicular angle, which destroys the belts (knock off the grits) compared to sharpening. A 1x42" belt has a surface area of 42" and a 2x72" belt has a surface are of 144"! Profiling knives wear the belts on my 1x42 faster, and since it has less torque, you have to push into the belt harder which knock off even more grit.