Expanding territory is so last century anyway. It just boost a national ego when looking at the map but does little to improve the standard of living of a people. More than 90% of China's population live east of the Heihe–Tengchong Line; less than 50% of the country territory.
https://en.wikipedia.org/wiki/Heihe–Tengchong_Line
Mongolia is 4-5x the size of Japan, but so what? They have little population and influence. 99% of the land are empty and inhabited. What's useful is really just the natural resources beneath the land. Trade and globalization means that you don't need to own the land to have access to these resources. What's important is if you have the money to purchase them.
So in today's world land is not as useful as institutions, technology and human resources. These are what really makes a country stronger and people's lives better.
Look at the US. It's not the largest country in land area in history, but it is certainly the most influential power ever. Not just because of her military, but also thanks to her culture, economics, institutions and technology.