Why does the self-inductance of a straight wire decrease when its radius increase?
Imagine a very thin long wire of inductance L. If it were comprised of two very thin long wires that were perfectly magnetically coupled (i.e. shared 100% the same flux) then the net inductance would still be L. That means that the individual inductance of each strand is 2L and, when paralleled, the net becomes L.

Take this a step further and have a large bundle of strands making up the long (but now not so thin) wire. Then ask yourself if one of the strands on one edge of the bunch will couple its flux 100% to a strand on the opposite edge of the bunch.