Mauritania dwarfs Oceania: about 1,038,352 km² km² against 8,600,000 km² km². At true size Mauritania is about 8.3× larger — the whole of Oceania fits into Mauritania roughly 8 times.
The old Mercator map treated both about equally (1.1× vs 1× inflation), so at least their relative sizes were preserved — even if both were drawn at the wrong absolute scale.
Mauritania fits 8× into Oceania.