Oh, so it was developed by SGI? ;-)
There's a PDP-11/73 about eight feet to my left; it's a 4U box, much smaller than a little SGI Onyx, let alone an Origin.
That said, there were PDP-11's that were desk-sized, such as the DEC Professional (about the size of an IBM 5150), or even smaller, the VT240 (which is largely an embedded PDP-11, and it has about the footprint of a C64, though somewhat thinner)