Economy of China is already 23% bigger than US economy, because the only measure of real output is GDP PPP and not Nominal GDP.
Prior to GDP and GNP, economic power of a country was measured by its industrial output. Economists and politicians compared steel output and coal output of different countries to measure their economic and potential military might.
Nominal GDP shows importance of Economy to international trade, business and investments and that is why it is the most commonly used indicator. Business, traders and investors don't care about GDP PPP, but care about the dollar market value of the economy (Nominal GDP).
However Nominal GDP, while showing market price of goods and services, doesn't show real output when adjusted for price differences for the same goods.
For example USA produced a Hamburger that costs 2$ and China produced a Hamburger that costs 1$. Both Hamburgers are the same, but Nominal GDP of USA will be 2$ and China's GDP will be 1$....So you will say that US economy is 2 times bigger than China's economy but that will be wrong, because both countries produced one same Hamburger and must have same economy size.
GDP PPP captures that fact and PPP adjustment shows that US economy is 2$ and China's economy is also 2$.
So in the example when US produced one Hamburger and China also produced one Hamburger, but market price of US Hamburger is 2$, while China's Hamburger is 1$:
Nominal GDP of US will be 2$ vs China's Nominal GDP of 1$.
But GDP PPP of USA will be 2$ vs China's GDP PPP of 2$-the same economy size and this is fare because both produced one Hamburger each.
Nominal GDP depends on national currency exchange rate. You calculate Nominal GDP of China by dividing China's GDP in yuan by the official exchange rate of yuan to dollar (121trln yuans divided by 7,19= 17trln dollars)
When national currency appreciates due to foreign currency inflows Nominal GDP increases and when national currency devalues Nominal GDP shrinks.
For example, Russia's Nominal GDP in 2014 was 2,6trln$, but when Ruble depreciated in 2015 Nominal GDP fell to 1,4trln$. So you can say that Russia's economy fell 1,85 times in a single year, but you will be wrong because most Russians didn't feel significant decrease in their standard of living during that period and GDP PPP remained the same.
China is a currency manipulator according to the US Treasury Department. China artificially devalues its national currency for the purpose of boosting exports and improving trade balance and increasing domestic employment, despite the fact that such policy decreases China's Nominal GDP. China's Central Bank prints yuans and buys dollars on the foreign exchange market and this devalues yuan relative to dollar and allows China's Central Bank to accumulate huge Forex Reserves.
If China stops being a currency manipulator, yuan will appreciate maybe by 40% and China's Nominal GDP will increase by 40% in a single year from 17trln$ to 24trln$...But if they do that, exports will fall, imports will increase, trade balance can become negative and there will a recession and rise in unemployment.
So the point here is that exchange rate based Nominal GDP shows market value of the economy to investors and traders, but doesn't show real economic output, which is showed by GDP PPP.
China's GDP PPP is 33trln dollars compared to 26,8trln dollars of US. It is like if China produces 33trln Hamburgers while US produces 26,8trln Hamburgers, so China produces more and its economy is bigger, even despite the fact that American made Hamburgers are more expensive when valued in dollars due to favorable dollar exchange rate.
Other indicators also show that China's economy is larger than US:
China produces 27mln cars, while US produces 10mln cars,
China produces 48% of ships in the world, while US only 0,35%
China produces more steel and cement than the rest of the world combined
China holds 33% of global manufacturing compared to US's 16%
However 20% of US economy is healthcare compared to 6% in China.
Also regarding India's GDP. I have been in India in Delhi, Calcutta, Mumbai, Agra and Varanasi and fare to be empirical observations led me to believe that they are lying about their GDP and GDP per capita.
If you look at statistics, then India's GDP PPP per capita is 9000$ - not far from Uzbekistan's 10.000$, so you might assume both countries are at the same level of development.
However, if you compare other indicators you can see the difference:
India:
electricity production per capita: 1262 Kwh
urbanization rate: 35%
new car sales: 1 car sold per 294 persons
Uzbekistan :
electricity production: 2000 Kwh
urbanization rate: 51%
new car sales: 1 car sold per 112 persons
How come Indians, who consume 60% less electricity, who are 1,5 times less urbanized and buy 3 times less cars have the same standard of living as Uzbekistan?
My feeling is that India is lying about its GDP and GDP growth rate and in reality their economy is 30-50% smaller.
Same for Egypt- been there and it looked like Arabic India:
Egypt 109mln:
electricity production: 218bln Kwh
per capita electricity consumption: 2000 Kwh
urbanization rate: 43%
new car sales: 250.000 cars
Iran 89mln:
electricity production: 353bln Kwh
per capita electricity consumption: 3966 Kwh
urbanization rate: 77%
new car sales: 1.050.000 cars
In no way, Egypt can have larger economy than Iran, when they produce 60% less electricity than Iran despite having larger population, have less urban population (47mln vs 69mln) and buy 4 times less cars.
So the point is that reality can be different from official statistics