All things being equal (mirror size, shape, lateral positioning, degree of "convexity", etc.) simple optics would seem to imply that the closer they are to the eye, the better.
My "reducto ad absurdum" argument is the mirror I use on my bicycle. It's mounted to my helmet and only about 1" in diameter - but it's so close to my eye I get a good rear view. That same mirror at a distance wouldn't work well at all.
If equally sized mirrors seem to work better when fairing-mounted, it must have something to do with their positioning, i.e. farther apart, clear of elbows, etc.