From a salmon anglers point of view
My take on large flies.
In days gone by before salmon anglers had modern fast sinking lines the only way to get a fly to fish deep was to have a heavier fly. In order to put more metal in the hook you made it bigger. This is why salmon anglers fish big flies in the spring as it was the way it was always done. I am beginning to think that 3” Willie Gunns are too big and think that maybe fishing a 1” waddington might be more successful.
Of course the size of the fly depends on the speed you are fishing it relative to the current, a small creature cannot swim against a strong current so a small fly must look natural. A bigger fly suggesting a bigger creature would be able to swim in a stronger current.