Revising the population down by 25% would actually help solve the math for the GDP also being grossly overstated. Both numbers are probably inflated.
https://thehill.com/opinion/international/5300578-china-economy-fraud-population/
Discussion
Unlikely to be less than 1.2b. China artificially lowers it's GDP through currency manipulation.
The underlying numbers that matter look great and continue to improve.
https://research.gavekal.com/article/unraveling-chinas-productivity-paradox/
Their only real concerning weakness is a declining real estate market