"With small servers, what you've typically got is one admin and maybe a couple of (volunteer) helpers."
It's not the numbers that matter, it's the ratio.
If a server has a thousand people and one moderator, that's 1:1000.
For Facebook to match that ratio they would need three million moderators, which they clearly don't have. Meta's entire employee count is only 70,000.
That's why massive networks depend heavily on automated moderation, they don't have enough people.