I imagine you could think of depth itself as a direct result of width in 2-channel playback. 2 speakers are already casting, according to their dispersion patterns, a given width into the room...where those patterns tend to overlap the most is where you get the most depth...yes?? So, to me, width is sort of the precursor, if you will, to depth...it's all the influencing factors of room, gear, etc that dictate how much or how little of depth or width in a given setup we can attain.
But if you are looking for some inherent cause, I would start with the dispersion capabilities of the speakers (perhaps especially the drivers) and, as well, the soundstaging of the associated gear and/or wiring. And it may take an unbroken string of an alignment of these factors to achieve a particular desired result - from source to room.
I've had speakers that, with the particular DAC I used to have, could image a full 5 and a half feet outside the cabinets, albeit at the expense of some depth, most especially at the rear corners. Now, I kinda have the opposite...only a foot or so outside, but with uncommonly good depth all around.
