Short answer : the whole cake will drastically disminish, focus on China’s part of it. Relatively to the world, China can be big as 25% of the world economy, as it was before Western countries’s boats started to look around.
Explained answer : realistically, the Chinese economy obeys the same rule as any other country economy : exponential growth and its limits.
From a certain amount of natural resources (domestic and imported), human and mechanical work (meaning domestic and imported energy), are used to produce more and more goods and services, e.g. growth. Growth every year means…exponential growth. And exponential output needs exponential inputs (mainly commodities and energy imports) without any negative loops. This allows food, industry and services per capita to growth exponentially as well, shown by GDP growth, even with all statistical bias and debt-driven wealth effects inside.
Exponential economic growth in turns allows exponential growth of population and pollution (« positive » loops). At one point, because of lower growth rate of available energy and natural resources, and more pollution, negative loop on arable lands, land fertility and death rate, economy slows down and collapse, later followed by population. This is called system dynamics and it looks like this :
For the whole world, the system dynamics fits to the world3 model scenario 1 called « standard run », with a notable difference in the stock decrease of available non renewables resources due to discoveries and huge investments for a few years on unconventional oil (now barely stopped). The economy slows from 2008 to around 2020, then collapses at a -30% rate by decade. Following, around 2030, death rate doubles in a decade and population falls, loosing 500 million people by decade.
Unfortunately, there is no country specific World3 model for China or any single country. We know China :
- is the first economy according to the IMF last report (20 933 billions of US dollars of GDP PPP) and represents 18,21% of the world economy
- represents 20% of the world population
- litterally eats half of the world commodities, particularly coming from Middle East, the Heartland (Central Asia), Canada and Australia
- already had the biggest population before exponential growth really started with Deng Xiaoping in the 1970’s
- has a huge pollution, reasonably causing the most important negative loop on health (e.g. death rate in a few years), arable land, land fertility and natural resources at country level
- will represents nearly 28 000 billions USD (GDP PPP) in 2020
- will have a population of nearly 1 384 millions of people (50 millions more) in 2020
China would normaly collapse like the rest of the world, which means at China’s gigantic level -30% in GDP and -100 millions of inhabitants each decade. However, China will likely collapse harder than commonly expected in the MIT’s World3 model, due to heavy negative loops from pollution.