The Carver Sunfire subs (they've been out for about 8 years) use ultra long excursion drivers (from the car audio industry) so they can make the cabinet so small. This also explains the high power ratings required to drive those subs in the small cabinet. These drivers use heavy cones and huge motors to overcome the tiny air space they perform in. The result is about the same as a larger (more efficient) driver & cabinet like you'd see 15-20 years ago.
Big bass in a small space is a valuable asset for both automotive and home audio markets, and people are willing to pay big bucks for it. Don't be fooled into thinking a fat surround automatically means a sub driver will make lots of bass. There are a lot of crappy drivers out there with beefy suspensions. A sub must also have the motor to drive the cone to the mechanical limits of the suspension - without melting or catching on fire.
I use a single Kicker 8" L7 (square sub) in a sealed box with 200Wrms. When it came out, the L7 was truly a monster 8" with 14mm of one-way throw. In my previous vehicles, I've run a single 10", two 10"s, three 8"s, and four 12"s. My next sub will likely be a JL 8W7, which is probably the best 8" sub in the universe, and imo one of the best subs, period.
