No, he isn’t.
Scale economics is the idea that producing 1,000 parts at a factory coats a certain amount per part, but there’s also a big startup cost, and various fixed costs (the warehouse costs the same to light regardless of how many machines you put in it, for example). If you did all the work to make 1,000 parts, you’re better off making 10,000 parts at the factory instead of making 10 factories across the country to make 10,000 parts.
@Dr.Strangelove, unless I mistook him, is not talking about that. He’s saying that if there are people doing a task, they will come up with new innovations every so often. If you have more people doing more of the task, they will come up with new ways of doing the task more often. And when they do come up with an innovation, it can quickly be applied to everyone, so the bigger economy gets more productive at a faster rate.
That’s a distinct idea from economies of scale (although the bigger society would benefit from that too).

No. We don’t have to assume that. There is no reason to assume that. It certainly has NOT been the case to date that multiplying a population by N over time changes productivity times N. Yes because innovation non linearly begets innovation no matter the absolute population size.
You guys are talking right past each other.
@Dr.Strangelove is saying that if there are two societies where all is equal other than population, the bigger society will produce more output. That’s obviously true.
You’re saying “any real pair of societies we look at where one is much bigger than the other had all kinds of other changes that impacted per Capita productivity”. Sure, that’s true. It has nothing to do with anything @Dr.Strangelove said.

I’m guessing it would be difficult to produce iPhones with a world population of 10 million.
It would probably be impossible, not just difficult.

To the point though, would that world overall be less happy because the iPhone was not around?
Yes.
You know that if you think you’ll be happier without an iPhone, you can just not get one? You don’t have to push for modern society to be abolished for everyone else, too.

Okay. We have a fundamental disconnect here. I don’t think we are as individuals happier because our tools and toys are one thing then or another thing now that was not even imagined of in the past. I strongly suspect my grandfather seeing a silent movie was as happy with the experience as I am seeing a movie on IMax.
Then why don’t you live in the woods like the majority of your ancestors since we split from chimps?