Mongolia dwarfs Oceania: about 1,560,428 km² km² against 8,600,000 km² km². At true size Mongolia is about 5.5× larger — the whole of Oceania fits into Mongolia roughly 5 times.
The old Mercator map treated both about equally (2.2× vs 1× inflation), so at least their relative sizes were preserved — even if both were drawn at the wrong absolute scale.
Mongolia fits 5× into Oceania.