No, it is *not* the same at different voltages, not by a long chalk.
The voltage drop for a given current at a given length *is* the same. Neglecting the issue of waste heat, this means that the voltage drops to zero at a much shorter distance with a lower voltage.
Example : 60A through 65mm^2, you are down to zero before 400m at 12V (ie 100% wasted as heat in the cable), at 220kv, you are less than 3% down at 200 km with the same cable.
the current will be higher at 12 volt but still much much less than20 amps
2.5mm^2 is unacceptable if someone is paying you to do the job.... The loss exceeds the 3% maximum allowable loss. But it is unlikely to be hazardous.