The illusion of soundstage.

Out of phase information on the record/cd will appear to come from beyond the boundaries of the speakers. In phase information will appear only between the speakers - unless you are getting a lot of side wall reflections which can give the illusion of a wider soundfield (you also lose some fine resolution that exists without the reflected information added).

Some manufacturers have made or altered equipment by playing with the phase relationships to make the soundstage seem wider than it can appear in a properly set up system of traditional equipment. This can be accomplished in speakers as well as electronics.

My understanding is that this is a complex question. Largely, the effect is a function of how the speaker interacts with the room. You can experiment with this by moving your setup around ...even into another room. Does it sound the same? Probably not. 

Toe-in of the speakers will narrow the soundstage, as will in my experience having the speakers too close to side walls. But, frankly I did not experiment much in this regard.

Have you shot your room with, say R.E.W.? That's a nice piece of freeware that runs on, say a laptop. You will also need a good quality microphone for this purpose ($50 or up), a microphone cable (usually a balanced cable with these mics) and an analog (balanced in) to digital (usb out) converter (I got one of several in the $200-500 realm. I think this is a very worthwhile thing for an audiophile to do unless you have far better ears than I have. You will thus have the tools to explore the invisible world of what your room does when it interacts to specific frequencies from your speakers. (I ended up moving speakers and pasting damping sobathane on the back of artwork, a mirror, the inside of a modern floor lamp,...)

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.

In my quest it has also been a give th and taketh away scenario. In looking for more depth I lost imaging width and settled for a middle ground. This was done moving speakers in and back and wider and narrower. So room and speaker interaction. The rest of chain certainly contributes but less. Except in my exp. tubes do seem to synergisticly encourage a bloomy boundarylessness that I like. 

Ivan,  5 and a half feet! How far out are sidewalls?