As it relates strictly to output, you'd be correct. Bigger is almost always better. But there is a lot more to the reproduction of low frequencies than simply output, not the least of which is the linearity of those frequencies. Bigger often means louder but there's no guarantee of it being any smoother. That's where multiple subs in multiple locations come in.The point I'm trying to make is that if you have a subwoofer and it's set up correctly the low frequency sound
would come from all around you leading me to question the need for two subs. Surely just one larger sub would
suffice ?
I believe @Flint has posted some charts in the subwoofer section showing how up to 4 subs (one along each wall) have the ability to offer the smoothest bass response in a given room.