Australia is clearly the larger of the two: about 7,706,009 km² km² against 3,155,807 km² km², roughly 2.4× at true size. Drag one onto the other on the map above to feel the gap.
Australia sits in Oceania, India 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 2.4×.
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.
India fits 2× into Australia.