Greenland is clearly the larger of the two: about 2,154,754 km² km² against 1,560,428 km² km², roughly 1.4× at true size. Drag one onto the other on the map above to feel the gap.
Greenland 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 1.4×.
The old map muddied it further: Mercator inflates Greenland about 11.8× and Mongolia about 2.2×, so on a Mercator-style map this pair appears 7.6× apart instead of the true 1.4×.
Mongolia fits 1× into Greenland.