More people means more ideas AND mutations
A core ideas in economics is that more people means more ideas. To take an extreme case, you would expect a population of one person to generate fewer ideas that a population of one million people. The precise relationship between population and ideas depends on factors such as the fishing-out of ideas, network effects, the composition of the population and the like, but it would seem to be strongly positive.
When you combine this assumption with the Malthusian concept that the level of technology constrains population, a larger population grows faster than a smaller population as a larger population generates more ideas to ease this Malthusian constraint. Michael Kremer used this argument to explain the greater than exponential population growth of the last million or so years (although that pattern has broken down since 1950).
This argument has a counterpart in evolutionary biology. More people means more mutations. From R.A. Fisher:
The great contrast between abundant and rare species lies in the number of individuals available in each generation as possible mutants. The actual number of mutations in each generation must therefore be proportional to the population of the species. With mutations having appreciable mutation rates, this makes no difference, for these will reach an equilibrium with counterselection at the same proportional incidence. The importance of the contrast lies with the extremely rare mutations, in which the number of new mutations occurring must increase proportionately to the number of individuals available. It is to this class, as has been shown, that the beneficial mutations must be confined, and the advantage of the more abundant species in this respect is especially conspicuous.
The greater number of mutations then provides more variation on which natural selection can act. Larger groups will, other things being equal, experience faster evolutionary change. Fisher again:
The theoretical deduction that the actual number of a species is an important factor in determining the amount of variance which it displays, thus seems to be justified by such observations as are at present available. Its principal consequence for evolutionary theory seems to be that already inferred by Darwin, that abundant species will, ceteris paribus, make the most rapid evolutionary progress, and will tend to supplant less abundant groups with which they come into competition. We may infer that in the ordinary condition of the earth’s inhabitants a large number of less abundant species will be decreasing in numbers, while a smaller number of more abundant species will be increasing …
Combining these two concepts - more people means more ideas and more mutations - gives larger human populations a double advantage over a long-term horizon. The higher level of production of ideas and beneficial mutations provides two avenues from which large populations can continue to grow.