Some people need the reality/ego check that full blown race 450s are actually making them slower riders, so many people think there 125/200/250 etc is holding them back and if only they had a cammed 450 they could have made that oobstacle and be soooo much faster.
Good ride buddy of mine runs a track and trail system up here where they do the WORCS race. He has a pile of Yamaha's, mainly YZ250 and YZ450F's. He got a lowered YZ125 a while back. Setting up a race course he rode it hard to time the laps. Then he thought it would be fun to do laps on all the bikes. Guess what, he was fastest on the 125. And he said it was by far the easiest to go fast on and took far less energy. It takes the right kind of rider but small light low power bikes can boogie through the woods. Very fast buddy Jake smokes on his husky 125 and refuses to add a 165 kit.