Good home speakers are not smaller. Those little cubes by Bose, JBL, etc are fine for low end sound, but if you want a good quality sounds system, you need big speakers. The small systems also use a bass cube to enhance the lows because the small speakers cannot hit that low without popping. One benefit of home speakers is environment, it is more controlled then a car, and you can encapsulate a speaker to produce the correct sound (ever play a speaker without it in a box, they sound pretty bad).
In a car, you have a little different environment, first you have extreme temperature differences, vibrations, etc that must be contended with, so you need to enhance the structure of the speakers. Second the acoustics in a car are very different, lots of metal and plastic to vibrate, so you need to design the encassments of the speakers to enhance the sound and produce the correct tones.
I know this was somewhat of a rhetorical post, but there is valid reasons why. If you ever want to test acoustics in a car (won't work in an SUV) with a standard trunk (not a hatchback), play a song with bass in it. Sit in the driver's seat and listen. Then open your trunk, sit back down in the driver's seat and play the same song over again. You will find that the sound is completely different, the trunk in most cars is actually one big encassment used to reverberate bass and sound.
