Oddly, if there was a huge length of wire at 400 Htz there would be slightly more voltage drop on that line...therefore the wire would have to be thicker. A bit theoretical that.
Not sure how theoretical. A problem with 400 Hz is that it easily induces currents elsewhere. Which means you have leak currents, you have proximity losses and skin effects... Those things get worse at low voltage - the lower your voltage, the stronger your current has to be, and therefore the stronger the induced currents.