hi! you may be interested in a few previous responses to related questions
Why does China have 1.5ish billion people but mongolia only has 2 million?
How did the Mongol Empire transitionned from being the biggest empire ever to be a small country of 3 million inhabitants.
How the did Mongols go for having one of the worlds largest empires to having just the developing country Mongolia is today?
How did Mongolia go from a powerful, huge spanning empire to a third world country in the modern era?