Misfit,
Drive size and wattage have an inverse correlation of sorts. As driver size gets smaller (for a subwoofer), you have to increase the wattage to maintain a similar SPL as a sub with a larger driver.
Take a look at some of the wattages used in the tiny Sunfire subs as an example. Sure they are small 9 inch cubes, but they need large amounts of power to make the sound.
You could drive a 36" woofer with about 5W and blow out your windows.

"Those who preach the myths of audio are ignorant of truth."