I disagree with that. The larger subreddits have significant moderation problems. Only through extraordinary efforts by the mod teams, such as at /r/askhistorians, are things kept in line. It's simple math: the more users you have, the more likely you are to have people posting in bad faith. If a subreddit of 1 million users has only 0.05% of its users posting low quality content, that's still 50,000 people that need to be moderated for.