Size has no correllation to the relative "speed" of the bass. An 18" plays every bit as fast as an 8" driver of equivalent quality. Many speaker mfgs will site cone weights or smaller cones as producing "faster" bass. In reality, in most cases, it turns out to be a lack of deep bass. Deep bass freq can be accentuated tremendously by a cars interior transfer function. You can gain 24-30 dB in a small hatchback. Whereas, in your HT, you are not accentuating anything but the very lowest freq. Now, to pile on top of that, many people who use ported boxes in a car, use a box building program that doesn't take this bass boost into account. When you have very deep bass without a gradual rolloff, you end with a tremendous unnatural bass boost. Lastly, you have UBLS (uncontrollable bass level syndrome) which prevents people from being able to control the bass levels.
To answer your question, proper understanding of teh effects of transfer function can help you design an enclosure for the vehicle that matches up well with the acoustic properties of the car. Over and above that, a nice parametric EQ for the sub can work wonders to tailoring the sound. The most natural sounding systems in cars I have heard normally have a 6 dB/octave rise in the bass response from 150 hz on down. Seems to get the bass level high enought to counter road noise and not sound too boomy.
Basicaly, knowledge, design, hardware, and most importantly implementation are key to getting the sound you want.
