Complex sentence: While they were still nearly as large as rheostat dimmers, they were highly efficient devices and their dimming effect was independent of the load applied so it was far easier to design the lighting that would be attached to each autotransformer channel.
Split into simpler sentences: While they were still nearly as large as rheostat dimmers, they were relatively efficient devices. Their voltage output, and so their dimming effect, was independent of the load applied so it was far easier to design the lighting that would be attached to each autotransformer channel.