deSilva
There is true proportionality, it's just that guys who state urban rules don't bother using their little grey cells to calculate what's really required and those that follow do so because they know no better.
Your 10A/sqmm ties in fairly well, though a little more conservatively, with the figure we used to use when winding transformers many decades ago. At that time we based current capacity on 1000A/sqinch for half-hard copper winding wire. At those sorts of loads there was no concern with heating of tightly wound coils so open air cooled wires should be more than happy.
jack