what was the reason behind Mongolia's population being very low in number?

Question

the fact that Mongolia has a population of only roughly 3 million people , yet it was a great empire in its history

Answer 1

In Mongolia's case, as with most places, geography is destiny. Check it out here:
http://education.yahoo.com/reference/factbook/mg/geogra.html;_ylt=AiKaMocp.XTAEivKb9gvzAa4ecYF

It's landlocked and most of the land is desert. The Gobi is one of the largest deserts in the world. Also there is only 0.77% arable land. Drought is a constant problem, as are fresh water and harsh winters.

It's not the kind of place conducive to much population growth.

Answer 2

Because long after Genghis Khan, Mongolia was under the rule of Manchu empire for 300 years. They basically tortured Mongolians and killed most of the population as well as turning men into monks so that they couldn't reproduce.