Well, think of it this way too:
A 10" woofer has a surface area of 246 square inches.
A 5" woofer has a surface area of 61.6 square inches.
You'd need 4 5" woofers to get the same surface area as a single 10" woofer!
Furthermore, larger woofers will have larger amps and will likely go to lower frequencies. If you like a lot of "PUNCH" (60-100HZ) then go with more smaller subs. if you want more BASS, get a bigger single sub.
Remember, subs are all about moving air, and the surface area of the sub has a lot to do with that, so all things being equal, it takes 4 subs that have 1/2 the diameter to have the same surface area as one sub, but everything is NOT equal, since the frequency response of the larger sub will likely be very different.