Translation:The population of China is the largest in the world.
Vi malpravas grandege ;)
Because that's their respective names: China, and the United States. If I'm not wrong, the article is only there when the name of the country includes a designation like "state", "republic", "kingdom"...
Here's a page I just found which explains it better than I: https://www.grammarly.com/blog/geographical-use-the/#:~:text=Fifty%20years%20ago%2C%20Argentina%20was,at%20its%20best.
Louis is right. In English we refer to China, without articles, unless we are talking about a particular aspect or condition. The China of Mao. etc. The United States gets an article because that name (and the USA) is short for "the United States of America"
Languages often get the article before them in Esperanto, but not countries. In English that's not always the case. And yes, it can be confusing to a native speaker also. I grew up calling one of the, until recently, Soviet states (and now independent country "The Ukraine", I now know that it's just Ukraine.
May I please ask which is your native language? Perhaps a better speaker of that language can explain things more clearly.