I agree with what you say about steel being stiffer than aluminum as a metal ---> http://strongframes.com/tech_geeks_l..._ID=12&copyID=

Aluminum is flexible compared to steel and fatigues whenever it bends therefore the design of the frame is engineered to overcome this flaw in the metal's characteristics.

The net result is a stiff frame made out if a flexible metal. Since it's less dense than steel the tubes can also be oversize as well to stiffen the structure up.

That link above is good reading.