I am not arguing about the size of empires. When Genghis Khan ruled Asia and half of Europe, he didn't loot it. you won't see magnificent palaces or temples in Mongolia. If you look at the data in the link below:-

https://en.wikipedia.org/wiki/List_of_regions_by_past_GDP_(PPP)

You will find Russia's GDP increased through the Mongol period (there isn't much granularity between 1000 A.D. and 1500 A.D., but there is a growth of 200 percent in that time period). 

You will also find who was the biggest empires through history. The Romans were about half the size of the Indian or Chinese contemporary empires. The Mughal Empire was the richest from the 1500s to the 1700s, when the British came. The Brits were not even the size of one of the Mughal provinces, and this was their golden age!