Nearly a tie: Mongolia and Saudi Arabia differ by barely -19%% in area — about 1,560,428 km² km² against 1,925,871 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 Mongolia about 2.2× and Saudi Arabia about 1.2×, so on a Mercator-style map this pair appears 1.5× apart instead of the true 0.8×.
Saudi Arabia fits 0× into Mongolia.