I've had a 15 inch ICON in my old explorer for the past two years and loved every second of it. But that car is now gone. I'm planning on getting a GCON that will go in a mustang. Obviously the trunk isn't nearly as big in a mustang, so I'm debating between a 10 inch or a 12 inch. The specs say a 12" optimal ported box is 3 cubic feet, and 2.5 cubic feet for the 10". These are bigger recommended box sizes than the icons of the same size...so why do the GCONs need such a big box? Just wondering cause I'm playing with limited room!

The Gcon can work quite well in Icon sized enclosures.  As it is our mid-power level SQ sub, and a little higher Qts, it is very clean and accurate in slightly larger enclosures.  Our SQ and home audio guys will often go with a Gcon in a larger box on modest power.  You can easily run a Gcon 12 in a 2.0 ported enclosure for sure, I do so often in my daily driver.