To be honest, the govt structure of the USA is simply not capable of governing a country with a population of 800 million. If immigration continues to increase, the USA will accelerate recession and fall into chaos.
The Chinese govt can rule 1.4 billion people because China is a single ethnic country, with the interests of the vast majority of its citizens converging. Moreover, Chinese culture values unity, abides by discipline and the law.
If the USA has too many people, it will only be a strengthened version of India.
The rulers of the USA are not foolish, they know this truth, which is why they control the number of immigrants.