Its not just about population. The UK was the world's foremost manufacturing nation at the time, just as China is now. It was the centre of manufacturing of an empire so the relevant comparison is with the population of the empire. There were no real alternative sources of energy - no nuclear, no solar, no wind (in a form suitable for most industry).

The British Isles were not providing food, heating, cooling, electric light, raw materials etc for the population of the British Empire.

And if you want to count the population consuming industrial goods as the population that "causes" those emissions, then China looks even better, because they are producing goods consumed by literally billions of people.