The Xixia empire, the smallest of these Buddhist empires, was 1,000,000 km2(390,000 sq mi) at its peak.
The size of the Mongol empire at its peak was 24,000,000 km2 (9,300,000 sq mi)! It was the second largest contiguous empire in history.
The British Empire was the largest. At it’s peak in 1920, it covered 35,500,000 km2 (13,700,000 sq mi),24% of the Earth’s total land area.