volume sub 12"

Hello, i'm sorry for my bad english! I just bought new alpine swr12d4 sub, i find some sub box and put sub in it. But those volume is more than recommended(1.25-1.50 cu. ft.). This box volume is about 2.1cu.ft,. Photo: http://www.bildites.lv/images/562lqre3ijf46w8i3hc.jpg So, what do you...