You're asking an extremely vague question that would require an extremely long and detailed explanation. The short of it is that it sound is created by the movement of air. The more air you move, the louder the sound will be. With bass it's a little more difficult to fill a large listening space with sound than it is a smaller listening space, which means if you are in a large listening space (like your van) you generally need to move more air to have the same output as a smaller listening space (like a compact car). If you took a 12" subwoofer and put it in compact car & measured it's output and then put that 12" subwoofer in your van and measured the output again, the output when in the compact car would end up being louder. The amount of air a subwoofer can move is called it's displacement, and it's determined by the size of the cone and the amount of excursion the subwoofer is capable of. So a subwoofer with a larger diameter cone, like an 18", is going to move more air than a subwoofer with a smaller cone, which means more potential output (assuming excursion and listening space are equivalent). However, large diameter drivers generally require a larger enclosure volume than smaller speakers, so you need to have more room available for the enclosure with a larger diameter subwoofer. In your van you have the advantage of having enough airspace to allow you to properly fit a pair of large diameter subwoofers, which is good. On the down side, and you also have more listening space to deal with when trying to create bass. If your goal was to create loud bass, your best option would be to use large diameter subwoofers with decent amount of excursion to create more air movement inside your large listening space which will allow you the ability to create louder bass. That's Bass 101 in a nutshell. Obviously there is much more to it, but that's the very basics in layman's terms.