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Thats what I found.... The steel wheel can be thinner because of the strength but the aluminum has to be thicker/bulkier to get the weight rating out of it... Look at the spokes for an example steel is like 1/4" thick while the aluminum is around an 1 1/2" thick so more material.
The aluminum ones are an inch wider than the steel rims too. They are 8" while the steels are 7". I found this out when I took the steel rims off of my truck.