joey

Let me preface this by saying this isn't how I think of it either, but here's the theory:

In the low-middle registers, your lips are more open. So the air flowing through is in a "big, fat column of air", creating a big sound.

To play higher, the aperture (opening of your lips) gets smaller, so the air moves faster in a narrower stream.

Or, in a slightly different variation: As you play higher your lips close. To keep them open, you must use a faster airstream.

To me, either way sounds needlessly complicated. Plus, it furthers the myth that playing in the upper register is vastly different than playing in the low and middle registers.

Joey