Remember, it's not so much the shape or size that was redesigned (if at all), but the spars, and other internal supports. The sources I posted are 1. Robert Grisnell, who researched the types thoroughly, and 2. Heinz Nowarro , who worked for FW. For example, they could have went with an I-beam shape instead of rolled steel spars, lessened and modified the other supports for strength, FW engineers haz lotsa branez 
It literally is first about the Moment of Inertia of the cross section when considering stress due to Bending loads (I.e Pressure Distribution span wise on the wing). It IS all about a.) shape (area distribution about the centroid, b.) stability under axial loads or bending loads, and c.) relative strength to weight ratio of the structural member
Stress = M*c/I where I = Moment of Inertia
As an example - For a given height of an I beam, you will increase the Moment of Intertia by increasing 'cap area' and maintain weight by decreasing the web thickness (after checking for shear stresses in the web). For the same geometry you will have the same Moment of Inertia if you change the material from 2024 aluminum to 4130 Steel and dramatically increase the allowable stress - but also pay a penalty of additional weight per linear foot.
A Rolled 'Hat Section' extrusion can give significant strength benefits but it has cost and logistics considerations - such as a.) it is not replaceable with conventional spar design without extensive re-design, and b.) as it does not use conventional and easy to source common extrusions or combinations of extrusion caps ("T" or "L" for example and riveted shear webs) the the air strike on your souece of specialized shapes may put you out of the wing business for awhile.
