Brian Solomon in his book Steam Locomotives (page 109 -- "Narrow Gauge Locomotives") says that prior to 1900 most narrow gauge engines were just smaller versions of standard gauge machines, and were built with the same arrangement of inside frames. The narrow inside frames, however, limited the size of the firebox and thus the power of the engine. Starting in 1903 D&RGW solved this problem by fitting outside frames to their new Mikados, which allowed the use of a larger firebox supported by a trailing truck. The larger firebox made these engines more powerful than an equivalent locomotive fitted with narrow frames and a smaller firebox.
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