China Shrunk Since the Yuan Dynasty...

http://www.artsmia.org/art-of-asia/history/images/maps/china-yuan-large.gif

In the Yuan Dynasty, the borders were much larger compared to present day China.  The emperor of the Yuan Dynasty was Kublai Khan, a Mongolian.  He and his people were outnumbered one to ten, but still managed to take over.  Since China in the Yuan dynasty included Mongolians, most of Mongolia and China was one country.