Search for 'enclosure alignment'.
basically the size of the enclosure affects its frequency response. A smaller enclosure might look better on paper, or in this case 'acceptable' on paper, but that does not mean they are truly recommending an enclosure size that would equate to a preferrable frequency response. Talk to Scott at Fi, the guy who designed them. He will tell you, they like really large enclosures. Id really like to try out a new XXX for home theater, as I could build a massive box. But something I need to fit in my truck? Nah, Id chose a different sub personally.