I agree with Nikolay on comma placement.
Oxides are produced when metals are in contact with air. Therefore, they only form on the outside surface of wires. Thinner wires have a larger proportion of their total mass exposed to outside air, and therefore, more of their mass will eventually become oxides.
If you use 100 kg of metal to produce a thick cable with a one-meter diameter, most of the metal will be inside the wire, and so it won't form oxides. If you use the same amount of metal to produce a very long wire that is only one nanometer thick, almost all of the metal will be very close to the air, and therefore almost the entire wire will eventually become oxides of the original metal.
If you're still confused, google "surface area to mass ratio"