I have heard this before...in what sense would adding population result in more innovation rather than having a more supportive environment for the people we already have (healthcare, education, financing, etc).
Population growth drives the competition for jobs, which encourages education as a means of achieving social status and a higher salary, which enables the research that produces knowledge. Highly unequal wealth distributions then enable wealthy individuals to spend vast amounts on moonshot projects that turn knowledge into technology. These things still happen without the population growth or inequality, just slower than when the pressure to advance is high. People without that pressure will tend to make more conservative choices about what to study or research, make fewer irrational investments, and spend less time working (because wages are higher when you're not in competition for jobs).
Simply more nodes for experiences to bounce through, so long as they have ways to communicate enough to copy, recombine, and incrementally improve on their activity. The more people copying, recombining, and improving on stuff, the more innovation.
Often easier to consider this in terms of competitive sports, though you could check seshatdb or something to see if it holds at scale. For example, YouTube really changed a lot with sports.
