Currently, the United States has the third highest population in the world, with 329 million peoples and also has the highest national aggregate GDP. The two countries that have a larger population than America, are China and India, in which, India will soon surpass China in population, and each of these countries have over 1 billion peoples in their nation, today. As of 2019, China is second to the United States in national aggregate GDP, and India is fifth. However, it is projected by the Center of Business and Economic Research that by 2033, China will have the highest national aggregate GDP, and India, which currently rests fifth in the world, will have risen to third. Additionally, it is projected by thoughtco.com, that by 2050, the United States will have slipped from third in population in the world, to fourth, having been surpassed by Nigeria.
The United States, declared its independence in 1776, and the Treaty of Paris was signed between the United States and Great Britain in 1783, in which, America's census of 1800 estimated that there were 5,308,403 Americans, at that time; whereas in 1801, Great Britain had a population of 7,754,875. Not too surprisingly, at that time, Great Britain was the greater power, yet, Great Britain, today, does not rank in the top ten of population, and is only seventh in national aggregate GDP, though, there was a time that this was "the empire on which the sun never set."
The main reason and the most significant reason why the United States will be surpassed in national aggregate GDP in the near future is because its population is considerably smaller than China, as well as being considerably smaller than India, and the numbers of the denizens within a nation, makes a material difference on how wealthy that nation is. In point of fact, while there is something to be said about GDP per person within a nation; power, that is, real economic power, resides in those countries that produce, consume, and export the most domestic product in total. That is to say, the countries that have the most economic might are in their way, the most powerful nations upon this globe, and a significant way that countries increase their economic might is the amount of population within that nation in conjunction with their knowledge and throughput, successfully applied.
The only possible and sustainable way for the United States to maintain its position as the economic GDP king would be to increase its immigration numbers substantially as well as to increase substantially its birthrate which has been declining for years. In actuality, though, the only way to increase GDP markedly in the here and now, is to increase substantially the immigration of those that are younger, because younger people are far more productive for far more years, especially as compared to those that are near retirement age, or are in retirement, who consume far less, except for things such as healthcare.
The United States is blessed as a nation with plenty of space to grow and plenty of food to sustain a significantly higher population but does not appear to be motivated towards increasing its population, though there has always been a very high correlation in regards to the increase of immigration, bringing more prosperity to this nation. The fact of the matter is, great nations need not just great people, but also great amounts of people, and the United States has placed itself in the position of being surpassed by China, simply because it wants to shut the door on immigration, and basically concede, without a challenge, its crown to another.