I don't have my mechanics of solids book at home, but I'll try and explain just by saying it is a physical property of the size of the two structures; the 2x3 is larger than the 2x2 and given the same wall thickness and overall length, the larger structure will have the greater torsional stiffness. Next time your in Home Depot, try twisting a 2x2 or 1x1 like a torsion bar, then try it with a 2x3 or a 1x2. The larger structure should be harder to twist. Even though they are solid, the same can be applied to the hollow steel tubing. Try and do a search on the net for torsional stiffness, you mau find a page with equations for different cross sections...