Another point to consider which was already touched on is that most car stereo systems are actively crossed over at least two-way's and bi-amped: Sub and mid/high's. Some are even crossed over 4 ways actively. Bi-amping like this can make a system rack up watts pretty quickly as well. Also, low frequency road noise does contribute to the fact that most people have to use much larger sub amps than in a comparable home system - there is a lot of competition in a moving car on the lower end of the freq. spectrum.

And as I think more on it, my yamaha rx-x2095 puts out 100w/channel or so x 5 channels = 500w RMS + a sub if I had one - Home systems wattage really isn't THAT much different than a car's, in the average cases. I would venture that in most cases anyone with a Dolby Digital setup with a amp'd sub would be pushing a minimum of 400 to 600 watts, which I'm sure would be higher than the average 4 speaker + sub setup of the car audio enthusiast.

Just thinking out loud:

Ben