It would not necessarily need the cross section of a 12 ga wire. There are 3 factors here- the resistivity per unit length of the conductor, the thermal dissipation coefficient of the conductor (which is a factor of surface area and insulation thickness), and how hot the insulation can get before failing.
For a PCB trace, the dissipation of a wide trace can be large with relatively little copper, and it is not covered with a great deal of insulation.
Is this a 20 amp continuous load, or intermittent? What are you trying to do?