I believe, and correct me if I am wrong as I probably am, that the steelies in the great lakes function differently than the NorthWest ocean going steelies.
If I remember correctly, the great lake steelies head to the great lakes, as if they were the ocean, and then back to the streams/rivers/creeks to spawn.
Meanwhile the NW steelies go to the ocean and return to the streams/rivers/creeks to spawn. (Which I am sure everyone already knows)
The fact that the NW steelies go out to the ocean is what helps to make them bigger. The ocean supplies a completely different ecosystem than the great lakes can. Apart from the fact that it is salt water system and holds a much larger variety of prey, the salt water itself affects the steelie's body in a completely different way on a biological level than does fresh water.
So, as far as a difference in size and why, I would say that it is due to the lack of saltwater in the Great Lakes Steelies.
As far as which rod to use, I couldn't really give you too much advice about that but I would really want to use at least a 7w. But that is me personally.