If only it were that simple dean
+1
On a rear wheel, a larger axle is undoubtedly stiffer and stronger….. but as, typically, standard QR’s aren’t snapping left, right and centre….and the amount of flex due to the constrint provided by the frame and hub (and the net tensile force in the QR itself) means you really are in the realms of “marginal gains”…
…. unless you are using a flyweight frame which itself might flex, so a bigger axle will stiffen the frame between the drop-outs, or you’re doing massive drops which might snap an standard QR.
On a fork, the argument is more persuasive as the fork bridge and the axle are the only two things stopping the (reltively flexible) fork legs moving relative to one another…. and the axle/hub will have more effect for less material being right at the end of the fork. Even then, the effect of using bolt-through axles is probably of equal benefit to increasing the axle diameter, with respect to stiffness of the fork legs.