China has 4 times the population. In any rational divvying up of the world's total emissions allowance by country China's share would be 4 times that of the US, but they are only emitting twice what the US is emitting.
Both are over their fair share, but the US is over by a larger factor so is farther behind on getting to where they need to be.
(This is not taking into account trade. Divvying up the world emissions budget by population gives the fair amount for each country if there is no trade. If there is trade the best way to handle it is probably to count the emissions for making things in country X that get consumed in country Y as being emissions in Y. With that correction China comes out even better).
Both are over their fair share, but the US is over by a larger factor so is farther behind on getting to where they need to be.
(This is not taking into account trade. Divvying up the world emissions budget by population gives the fair amount for each country if there is no trade. If there is trade the best way to handle it is probably to count the emissions for making things in country X that get consumed in country Y as being emissions in Y. With that correction China comes out even better).