A larger population means bigger numbers on the everything that's on a normal distribution, like intelligence. That one genius in a 100 million? With 1B population, you only have 10 of them. With 8B population, there are 80. That's 8x more chances one of them does or finds something revolutionary, 8x chances that person is not perishing somewhere in a cobalt mine as a kid. It slashes times waiting for a genius to appear 8x.
A bigger population also enables extreme specialisation in very niche fields. It enables bigger recruitment pools for complex processes and manufacturers like, for example, ASML.
It's, in a short word, massive parallelism.
Now, a counter-argument to everything I said is looking at the demographics of geniuses, revolutionaries, specialists and noticing most of them were well-fed, well-educated and raised in stable, liberal societies (liberal as opposed to, say, taliban-like societies). So, in theory, we could get by with a smaller population, if it was a rich, western-like, population. In that case, the only constraint would become genetics; so an 100M population would have to wait a couple of generations for their genius, instead of a 8B pop potentially getting them all at once.