Perceived width is actually an aberration - so when image focus improves, the sensation of width diminishes. Its akin to looking at an image out of focus, then seeing it suddenly in focus - the size of the image gets smaller but you can see things much more accurately. Another way of looking at is perspective. If an instrument gets deeper into the sound stage, it naturally goes back and apparently decreases in width. So when you improve instrument placement focus in the sound stage, the perception of width will decrease.
There is an exception to this rule, in that you can encode sounds to sound wider than the loudspeakers, but this effect (replicating the phase delays and resonances of the earlobes and changing the left right phase) can increase width beyond the loudspeakers - it can also be used to encode height. But these effects are very rare and a bit hit and miss. Its these resonances and phase delays that allow binaural recordings on headphones to work. In this case, when image placement improves, then you get an increase in width and height - but as I said, these effects are very rarely found.
Rob