Total output is what really matters, not just the per capita. The earth has a carrying limit for total emissions.

The EU has nearly 200 million more people than the US, roughly 60% more, squeezed into an area less than half the size of the US. Whose fault is that population density? The total EU emissions are ~80% as high as the US in ~45% of the land area (a drastically worse rate of emissions per km2), due to that population density. How do you think that emissions concentration impacts the local environment? Population matters, it's why we wouldn't have to care much if Iceland had 2x the per capita emission output of the US.

If your nation has three billion people - ie vast over-population - you shouldn't have the same per capita emissions considerations or concerns as a nation with three million. I understand that sounds unfair. The earth sets the real limitations and it doesn't care about perceived fairness.

If a country like Nigeria or Indonesia somehow rapidly expanded to five billion people, would it be acceptable for them to have the same per capita emission output of the EU? Obviously not. It'd be an environmental disaster for the planet due to the overall output. Population is the other critical factor in emissions (eg in China's case their emissions would be catastrophic at US per capita levels with 1.4b people in that small of an area).

Only means to decrease total CO2 output is increasing efficiency per capita CO2 output captures this very well. Population will not shrink anytime soon and moving people around at scale is not feasible.