Nearly a tie: Indonesia and Mongolia differ by barely -17%% in area — about 1,872,242 km² km² against 1,560,428 km² km².
Both sit in Asia, so this is a same-region comparison: the new Equal Earth map draws both with the same honest geometry, and the real gap is exactly the 0.8× you see above.
The old map muddied it further: Mercator inflates Indonesia about 1× and Mongolia about 2.2×, so on a Mercator-style map this pair appears 1.8× apart instead of the true 0.8×.
Indonesia fits 0× into Mongolia.