The strength of a material, steel in this case, is determined by it's tensile strength and is expressed in pounds per square inch. The size of the "A" frame members might be larger due to the specific loading in that area.
Think of it this way, on a car with unitized construction, the members that support the wheels are of a larger, different shape than the member that runs under the cabin. They can make the member under the cabin smaller or different shaped because the roof structure, door pillars, etc. all add strength to the cabin assembly even though they are made of steel the same tensile strength.
2006 23B Hybrid with 10k round bar WDH
2011 F150 4x4 SCREW Ecoboost, Max Tow, Integrated TBC, 3.73 LS axle, Firestone Ride Rite airbags.