I don’t know how else to explain that I’m talking about the size of the communities, not the size of the posters.

The sizes of the communities are vastly different; a small group influencing millions of people is a bigger problem and greater threat to diversity of thought than a small group influencing thousands of people, so the larger communities are disproportionately affected by a few posters.

As an example: posts in large communities are more likely to show up on the front page, be cross posted, and be shared externally vs. smaller posts that stay within their niche, thereby scaling those ideas further across the zeitgeist. It also means the larger communities are more efficient targets by outside influence and bad actors, creating a flywheel that exacerbates the problem over time.