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Xmax and surface area are the two biggest factor in how loud a sub can get. Your average 10" and 12" have around 12-14mm of xmax, 15" has around 8-10mm of xmax. Then you have your "super subs" such as the XXX and Brahma that have 32mm and 28mm, respectively.

if boils down to the efficiency of the speaker.
a speaker can require a lot of power and actually return very little in actual work.
most speakers only return about 5% of the power put into them as sound coming back out.

you can have a sub that's over-damped by having too large of a magnet structure (see audiobahn) so it looks all massive and impressive, but in actuality it just sucks up power and doesn't perform nearly as well as similar better subs.
Then you can have a very efficient sub with low cone mass, foam surround, etc and it can use less power to move equal air.

in short, that's essentially how a sub can be louder with less power handling.
make sense?

Actually, that does make a lot of sense. Thanks for helping me understand that concept better.