Guys, I was wondering why some people go outside the recommended cubic feet from FI for enclosure size? I am thinking because its vehicle specific or is there another reason? I have seen guys running 4 BL 12's in a 13 cube net enclosure.

Larger = increased efficiency at tuning 

bigger boxes are typically more efficient, but you can run into softpart limitations 

Usually because people read what Julian typed and think they can make a gain that they won't. There are some justified installations that way, but 99% of the time it is someone who thinks they know something that they actually don't understand.