Nearly a tie: Mexico and Mongolia differ by barely -20%% in area — about 1,962,279 km² km² against 1,560,428 km² km².
Mexico sits in North America, Mongolia 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 0.8×.
The old map muddied it further: Mercator inflates Mexico about 1.2× and Mongolia about 2.2×, so on a Mercator-style map this pair appears 1.4× apart instead of the true 0.8×.
Mexico fits 0× into Mongolia.