I would add to Sean's thoughts that a larger woofer, with its greater diaphragm area, also doesn't have to make as large an excursion as a smaller woofer would. The more cone area you get - whether by using multiple smaller woofers or one larger woofer - the less excursion is required to reach the same SPL. A larger woofer also can (up to a point) use a larger motor/coil assembly, which can handle higher power (dissipate more heat), just like multiple smaller ones. Point being, either way you go (bigger woofers, more woofers), the method and goal are the same: Increase total cone and motor size, along with cabinet size, and it will give increased bass capacity.