No, he doesn't get shorter as he plays it, I just suck at screencaps
Chainsaw Clarinet Mashup Instrument: B Sharp Indeed!

Because that’s what friends are for, this is a video of Nathan Pryor’s friend testing the chainsaw clarinet Nathan built him, which combines his two biggest passions: playing clarinet, and…chainsawing? Hey, different strokes for different folks, that’s my motto. My favorite stroke? The breaststroke. You know why? “Because it has boobs in the name?” What? No — because you look like a frog when you do it. You know you really need to get your head out of the gutter and– “In-between your boobs?” Haha, you got me! Permission to motorboat away!