From their blog - 
We constantly get asked what size tires fit in our fatbike frames. The easy answer is four-inch tires on 82mm rims. This is what the Mukluk and Beargrease frames are designed around. Usually the next follow-up question is: What is the biggest tire I can run? The answer to that is: It depends.

The Beargrease and Mukluk frames are designed around a 170mm-wide rear hub to facilitate fitting the wider rims and tires that make fatbikes… well…fat. The 170mm hub spacing isn't an accident. It has roots in the originalSurly Pugsley frame design.
When Surly was designing the Pugsley, they knew they had to design a frame that would allow the chain to pass by a Surly Endomorph tire on a Large Marge rim. This combination of rim and tire measured out to about 91mm at the widest point. Surly also knew that they were using a MTB Triple crankset mounted on a 100mm bottom bracket, which has roughly a 65mm chainline. This positioned the granny ring at 56mm from the frame's centerline.
Through some simple math and sketches, it was calculated that if a 135mm standard MTB hub was offset by 17.5mm towards the driveside of the bike, and the chain was placed in the smallest (or lowest) gear combination (biggest rear cog, smallest front chainring) that the tire capacity produced by the chain around the critical area of the tire was around 94mm. This accounted for a few millimeters of clearance and alignment tolerance. At the same time, the alignment of the cassette to the chainrings was reasonable, not that much different from a regular mountain bike, so shifting performance and drivetrain wear would not be affected. This all seemed pretty reasonable, and thus, the Pugsley was born and fatbiking for the masses changed forever.

Let’s get back to the 170mm rear spacing thing. If we look at the rear end of a Surly Pugsley frame with 135mm spacing offset 17.5 mm to the driveside, and calculate the symmetrical equivalent, we come up with 17.5 + 135 + 17.5 = 170mm. This is why most fatbikes today use a 170mm wide rear hub. It results in the same cassette alignment as the original Pugsley, which we know offers decent space for fat tires and rims, and also has an adequate chainline.