It's not huge, it's a mid-tower (admittedly, not a pretty one). But the real benefit is that it is upgradable. Basically you are trading off user serviceability for "it just works" and the form factor.

Another thing the Steam machine has is HDMI-CEC support, which is nice if you intend to use this with a TV, perhaps with KDE Plasma Bigscreen. But $1000 is rather steep for a console/HTPC.