Moderation cuts two ways. Yes, bigger servers have more posts and more posters to moderate. But they also, being larger, can field a well-trained moderation team.
With small servers, what you've typically got is one admin and maybe a couple of (volunteer) helpers. Maybe they are good at what they do. But maybe they're erratic, distracted, or just not suited to the task. It's quite difficult really, and more complex than one might think.