If you take a good look at the economy of these countries, America and China, you will see how strong their man power is. The development of a country mainly depends on the labor force, the country and nation that are hardworking can develop very easily. Countries that have large populations earn millions of dollars a year by exporting workers.
There is no need to be surprised by this because the residents of the two countries you mentioned are residents who do have the intention to work and try to make their country more advanced by helping the government in realizing this. Because when all the people in the country are seen to be more prosperous through the efforts they have made so far, of course the country will look advanced and more developed in terms of economy so that people in other countries also need to follow in such footsteps as long as this is still very possible to do.