In much the same way the mountain bike industry is messing around with wheel sizes to squeeze more money out of us give us the best possible ride, the mx world has done more or less the same. The experimentation has largely been with front tyres from 23″ down to 18″. 21″ is the norm now and it’s based on simply finding a wheel size that will roll over rough terrain best without compromising steering ability. Rear tyre size has remained largely the same to simply provide the best possible traction and rear end stability. It’s just through experimentation that we have what we have in MX.
^ This pretty much. Front wheel is larger diameter with a narrower width (and shallower tire) to give good roll over and cornering/handling. Rear wheel takes more of a beating put also needs to provide traction so the wheel is smaller to allow for a deeper and wider tire to be used.
Some people have been saying that having a larger 650b front wheel and 26″ rear wheel allows for the best MTB setup, possibly through similar thinking to mx bikes, though obviously the forces involved are significantly reduced.