Australia dwarfs Indonesia: about 7,706,009 km² km² against 1,872,242 km² km². At true size Australia is about 4.1× larger — the whole of Indonesia fits into Australia roughly 4 times.
Australia sits in Oceania, Indonesia in Asia. On the old Mercator map the latitude gap distorted how large each felt beside the other; at true size the answer is simply 4.1×.
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.
Indonesia fits 4× into Australia.