theres a few different diameter wheel spindles.
5/16" 3/8" 9mm 9.5mm and 10mm. Then theres the even thicker through axle wheels, but those will no way go into the more regular fork ends
some bikes have a thinner diameter axle on the front wheel
Cant understand that. Ok there might be a gram or 2 weight loss. But the factory has to get different size axles, cones and nuts. Would be cheaper if they stuck to one size through out. Also easier for the assemblers. Would need less tools.
Ive filed fork ends on old Raleighs. They use the thinnest axles. With a key hole shaped hole in the end of the fork. I wouldnt recomend filing the axle. Apart from loosing strength. It would mean that if you needed to change the wheel, you would have to file that too.