Here's some additional information from Guide to North American Steam Locomotives (pages 158-159): " The difficulty of scaling down a standard gauge locomotive was that the power output was similarly scaled down, and the most severe limitation was firebox size. A firebox between the frames of a narrow gauge engine couldn't be much wider than a coal shovel; placing a firebox above the frames helped, but it was still limited by the back-to-back spacing of the wheels. Moreover, space between the frames was too restricted for easy maintenance of journals and valve gear." ..... "By 1903 locomotive technology had progressed so that Baldwin could build 2-8-2s for D&RG with twice the pulling power of the Consolidations. The locomotives were Vauclain compounds, and they had outside frames. The trailing truck supported a wide firebox entirely behind the drivers, and the wider spacing of the frame rails made the locomotive more stable than an inside-frame model."
So outside frames offered a variety of advantages. But accommodating a wider firebox was still the basic reason for adopting outside frames.
