Nearly a tie: Mongolia and Sudan differ by barely -16%% in area — about 1,560,428 km² km² against 1,863,027 km² km².
Mongolia sits in Asia, Sudan in Africa. 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 Mongolia about 2.2× and Sudan about 1.1×, so on a Mercator-style map this pair appears 1.7× apart instead of the true 0.8×.
Sudan fits 0× into Mongolia.