So you have to count the EU as a single country to push China into third place. This is something people argue about. During Brexit I many people assured me the EU was not a country.
If you use PPP rather than nominal GDP China is actually the biggest and the EU the third biggest.
Also worth noting that, despite expansion, the EU was a smaller proportion of the worth economy before the UK left, than the EEC was after Britain joined. The trend is very much away from the West.
Their population is stagnating and there's some good signs that the numbers they gave were not accurate in the first place, China's real population is likely lower than India.
what about population? as that's the main driving factor for pollution - especially if you couple that with energy needs.