Nearly a tie: Mexico and Saudi Arabia differ by barely 2%% in area — about 1,962,279 km² km² against 1,925,871 km² km².
Mexico sits in North America, Saudi Arabia 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 1×.
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.
Saudi Arabia fits 1× into Mexico.