Because you are paranoid and also have a small apartment. I have a mini-itx home server build with 16 gb of ram and 4 WD 3tb RED drives. It doesn't run ZFS because I don't have ECC ram in it, but it's the smallest computer I could buy that would fit the hard disks I already owned. If I were starting from scratch I'd probably use 2.5" drives instead, and if I wasn't working for a non-profit, I'd probably use SSDs now that they are about $.27 a gb.
Mini-itx I can understand, but this box has one drive of 1.5TB max (according to http://www.hp.com/go/Z2mini). You would have to connect any other drives externally. If you do that, the result probably takes more room than that mini-itx.
