1 US citizen is worth about 5 PRC citizens in terms of economic activity. The PRC's GDP per capita is roughly the same as Brazil's (this situation is unlikely to improve either [1]). Imagine Brazil scaled up to 900 million people and 1/3 of them are over 65. Social welfare systems start to buckly/collapse, and retirement benefits must be cut or the retirement age raised.
The only thing the PRC could do right now is force people to reproduce at gunpoint, and they even shot themselves in the foot there with the One Child Policy and now having 30 million more young men than women.
It's going to be scary to see Japan, the PRC, the ROC (Taiwan), etc. lose 1/4 of their populations in the next 30 years. The US is the only developed country with okay demographic projections through 2050, and that's only because of immigration. Not sure where you're from, but if you're in a developing country you'll probably be doing okay in 2050.
So the USA can dominate the entire planet and most of space with 330 million, but China is dead in the water if they drop to a mere 900 million?
If you look at China's problem from a third world perspective like mine, ALL of their issues get easier with less people.