Mike,
The need for larger N.G. engines was one reason Baldwin (at least) went with the outside frame. The outside frame gave the design engineers a wider base upon which to build the larger fireboxes and boilers needed for these new narrow gage "monsters". The outside frame also made these larger engines more stable. Along with the outside (hence wider) frame, the counter weights were also hung on the outside and, along with all the main and side rods, this helped to offset the engine's higher center of gravity (due to the larger diameter boilers and wider/taller fireboxes).