Is the main frame of the locomotive INside or OUTside of the drivers? As they started building bigger and bigger power for narrow gauge (especially in 3' gauge, but also seen in 2' guage), they needed to increase the stability of the locomotive. By extending the axels outside of the drivers, they could place the frame out there and provide a wider "base" on which to construct a locomotive. Usually, the counterweights on each driver are still located OUTside the frame at the end of the axels to allow adjustment. Look closely at those photos, and you will find that on the OUTside frame locomotives, it is just the counterweights that you can see spinning around out there.
Bill