Australia is clearly the larger of the two: about 7,706,009 km² km² against 17,840,000 km² km², roughly 2.3× at true size. Drag one onto the other on the map above to feel the gap.
The old Mercator map treated both about equally (1.2× vs 1× inflation), so at least their relative sizes were preserved — even if both were drawn at the wrong absolute scale.
Australia fits 2× into South America.