Size of load
Carrying capacity is determined by weight of track, the capacity of the rolling stock and bridges, and the like. One thing a broader gauge gives you is the ability to transport a wider load at speed, and still maintain stability. The Maine 2-footers maxed out at about 8' wide, and were usually narrower. Anything wider would require moving very slowly and carefully, lest the train tip over. A 6' gauge train, on good tack, could transport wider cargo than a comparable 4' 8 1/2" gauge train, all things being equal.
Broader gauges tend to need broader curves, also--and that allows longer pieces of cargo. Curves, of course, can be made wide in any event.
But, broad gauge is much more expensive to build, though not much more costly to run.
Break of gauge is often a problem, changing cargo from one train to another. Sometimes narrow gauge to standard is less of a problem. The East Broad Top, for example, was a coal mining narrow gauge railroad. Coal was loaded into its cars, and had to be unloaded at the crushing/processing plant. It was then reloaded into standard gauge for distribution.
