The Mongol Empire was larger than the Roman Empire at its peak. The Mongol Empire, established in the 13th century, covered about 9.15 million square miles, stretching from Eastern Europe to East Asia. In contrast, the Roman Empire, at its greatest extent in the early 2nd century, spanned approximately 2.5 million square miles, encompassing territories around the Mediterranean Sea and parts of Europe, North Africa, and the Middle East.
While both empires were incredibly influential in their respective eras, the sheer landmass controlled by the Mongol Empire far surpassed that of the Roman Empire. The rapid expansion of the Mongols under Genghis Khan and his successors allowed them to unite a vast territory, creating a sizable empire that facilitated trade and cultural exchanges along the Silk Road.
In summary, when comparing the two, the Mongol Empire clearly holds the title for being the larger of the two in terms of territorial extent.