"Why is a mouse that spins?" is probably a nonsense riddle, like "Why is a raven like a writing desk?" from Alice in Wonderland. That is, it's funny because it isn't funny. The answer is also nonsense, "The higher the fewer."
There is another theory. According to this theory, the "mouse" is the governer on a steam engine. As it rises, it slows the rpms of the engine, thus the higher the mouse the fewer the rpms. I don't know if this is a real explanation, or an after-the-fact rationalization, like the one about "notes" of ravens and writing desks.