Argentina is clearly the larger of the two: about 2,782,703 km² km² against 1,560,428 km² km², roughly 1.8× at true size. Drag one onto the other on the map above to feel the gap.
Argentina sits in South 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 1.8×.
The old map muddied it further: Mercator inflates Argentina about 1.5× and Mongolia about 2.2×, so on a Mercator-style map this pair appears 1.2× apart instead of the true 1.8×.
Mongolia fits 1× into Argentina.