Mexico dwarfs United States: about 1,962,279 km² km² against 9,439,424 km² km². At true size Mexico is about 4.8× larger — the whole of United States fits into Mexico roughly 4 times.
Both sit in North America, so this is a same-region comparison: the new Equal Earth map draws both with the same honest geometry, and the real gap is exactly the 4.8× you see above.
The old map muddied it further: Mercator inflates Mexico about 1.2× and United States about 2×, so on a Mercator-style map this pair appears 7.9× apart instead of the true 4.8×.
Mexico fits 4× into United States.