Question: How many posts do you view from a given subreddit in a day?
How many comments on any given post do you read?
How many people need to be in a community to generate that many posts and comments? Because communities don't really need to be any bigger than that.
Having FOMO over what's going on in other communities is normal, but it's not helpful or useful. It's not like you weren't missing out on 99.9% of posts on big subreddits, or 99.9999999% of comments. And let's not pretend like what floats to the top in large subs is the best content. Popularity contests are not meritocracies.
Having 1000 communities on the same topic with 1000 active users each is better than having 1 community with 1,000,000 active users. Those users are easier to moderate, they're more likely to see other community members as people, and they're more likely to have their posts and comments viewed and interacted with by others.
And if something really interesting is happening in another one, someone will link to it.