Dave,
The relationship between conductor cross section and ampacity is nowhere near linear. If you look at tables of ampacity, you will notice that the cross section increases far more rapidly than the current being carried.
The factors to consider include the heat generated in the conductor, the thermal resistance to heat being carried to the environment, the temperature rating of the insulation, etc.
If you maintain the same relationship of copper cross section to current, then you get the same 'current density' and thus the same heat production per amp. But in a round wire, the cross sectional area goes up as the square of the diameter, while the surface area goes up linearly with diameter. This means that if you were to double the wire diameter, the cross section to carry the current would go up by a factor of 4, but the surface area to dissipate the heat would only go up by a factor of 2. If you tried to keep the same current density, you would push 4x the current through the wire, and get 4x the heat production, but not have sufficient surface area to dissipate the heat. So you need to reduce the current density, so that the increase in heat production in a given length of wire will match the increase in heat dissipation capability.
-Jon
