Why same size subwoofers?

[laptopbuyer]

If people are buying more than one subwoofer, why do they always get the same size cones? Why not get like an 8, a 10, a 12, and a 15 inch subwoofer to get the benefits of all of them?

 

[R_1]

it has to do with the frequencies that the subwoofer reproduce and how those frequencies carry.
Low frequencies are hard to pinpoint the location, but a cymbal crash will make us turn to the source - the frequencies are far more directional. more Subwoofers in a small space are not useful.
the larger subwoofers can recreate lower frequencies, but paring a 12 inch with an 8 inch would be wasteful. the 12 can reproduce all the frequencies the 8 can. so the 8 paired with a larger unit may as well not be there. it will be overshadowed by the larger unit with the lower range. its the nature of low frequencies.
2 subwoofers of the same size can accent each other without overpowering the other.

this is how I understand the situation

 

[americanaudiophile]

Different size woofers will respond differently to the same signal.
In general smaller woofers can move faster than larger ones. So if you have different size woofers reproducing the same range you get the strange result of the same sound being reproduced differently.
You can use different crossover frequencies so the smaller woofers play the higher bass range which may benefit from a faster cone that will product more detail and the larger woofers can move more air and don't need to be quick.