I'm looking at the Surly Krampus and Karate Monkey. It looks like the difference is basically that the Krampus is longer and fits bigger tires. But the Surly website says the Karate Monkey can fit both 27.5 and 29 wheels, but the Krampus can only fit 29. Why would that be? With disk brakes, shouldn't it always be possible to fit SMALLER wheels? I'm confused.