That's not true. 

When the Alminum is used, the thickness of the blade must be thicker since the strength is low, and thus the flow rate is limited, and the volume must be increased because the Back Disk must be thick. On the other hand, a thin blade can be applied to stainless due to its high strength and the weight can be reduced by making the back disk thin. Therefore, in a rotating body, weight is not a matter of material, but a matter of design.