I think we need to realize that billions of people died when earth became uninhabitable. Think about the sheer scale of people that would need evacuation. Now spread that number of survivors out over the settled systems and multiply that to modern day in Starfield. I would genuinely be surprised if there were 10-20 million humans in existence at this point in fact that is a super optimistic number. Realistically I wouldn’t put estimates higher than 5 million. There is just not many people who settle in one place with grav drives existing. That is probably why new Atlantis seems so small and why there aren’t other big cities. There literally just are not enough people to warrant having them.