Australia dwarfs Mexico: about 7,706,009 km² km² against 1,962,279 km² km². At true size Australia is about 3.9× larger — the whole of Mexico fits into Australia roughly 3 times.
Australia sits in Oceania, Mexico in North America. On the old Mercator map the latitude gap distorted how large each felt beside the other; at true size the answer is simply 3.9×.
The old Mercator map treated both about equally (1.2× vs 1.2× inflation), so at least their relative sizes were preserved — even if both were drawn at the wrong absolute scale.
Mexico fits 3× into Australia.