Nearly a tie: Democratic Republic of the Congo and Mexico differ by barely -16%% in area — about 2,334,703 km² km² against 1,962,279 km² km².
Democratic Republic of the Congo sits in Africa, Mexico in North America. 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 Mercator map treated both about equally (1× vs 1.2× inflation), so at least their relative sizes were preserved — even if both were drawn at the wrong absolute scale.
Democratic Republic of the Congo fits 0× into Mexico.