> Except of course the problems caused by insufficient population. That is: insufficient production of food

That does not make much sense as this is a relative limitation. If you have twice as many people you also need twice as much food. If the manpower is the problem of food availability (which it isn't really, but let's say it is) then you need to increase the fraction of people working on it, not just the absolute number.

In contrast, emissions have an absolute effect on the climate. Twice as many people doesn't mean we get to emit twice as much for the same effect. The carbon budget is per planet, not per person.