It's quite possible that China's population has also been significantly exaggerated. One thing that the China actually does quite well is universal childhood vaccination for certain diseases. Independent researchers looked at the number of vaccine doses ordered and found that it's way lower than the official government birth statistics would suggest.

https://www.pewresearch.org/short-reads/2022/12/05/key-facts...

Now this is an unproven "conspiracy theory" but it's entirely plausible that corrupt local officials have inflated population numbers in order to be able to embezzle more of the funds that flow in from the central government. A lot of people might exist only on paper, but it's impossible for outsiders to precisely quantify.