Well, I have to say that though I accept the science for what it is, it still seems counter-intuitive to me.
It seems to me that a 12" woofer is moving a lot more air than a 1" tweeter. As such, the woofer's mechanical efficiency (and at some point frequency response), is more greatly affected by air density.
Sound pressure level is a measure of the power (energy per unit time) transferred into air (as pressure waves) by the motion of the speaker diaphragm. Energy (or work) is force times displacement, and if we count energy per unit amount of time (i.e. 1 second), we get power.
Comparing a woofer moving at say 20 Hz to a tweeter moving at 2 kHz, the tweeter cycles back and forth 100 times more than the woofer. So the travel of the diaphragm doesn't tell the entire story. It must also be multiplied by the frequency (number of repetitions).
Moreover, force is mass times acceleration. Thus, the higher the frequency, the higher the acceleration and therefore the higher the force (derivative of sin ωt = ω cos ωt).
Combining both effects, the amount of power transferred into the air, when diaphragm travel and diaphragm area are kept constant, increases with the square of frequency. This is why we can have small tweeters with less diaphragm travel producing the same SPL at high frequencies as large woofers at low frequencies, as both transfer the same amount of energy into the air.