I remember some years back on Reddit there a change to what the default subs were. Suddenly my reddit frontpage was full of /r/aww and a lot of incredibly low effort content. From then on it was necessary to show only subs I subscribe to. I like memes as much as the next person, but in moderation.

I'm not worried and don't see a big problem if there are some subreddits/communities with memes and low effort content.

In the discussion of size. I subbed to the classicalmusic sub. Right now there is an equivalent in the fediverse I think but it's quite dead. Just having one community for all types of music isn't really appealing. I don't want to go through 1000s of popmusic post to find two about classical. This example of mine with classicalmusic transfers well to other topic. I started using a distro of Linux on my computer and want to connect with others also using Linux. For all these niche interest size is important.

Then as for joining. I won't say it's complicated to sign up, but there is an extra hurdle in a way with how it's a lot to take in about how the fediverse is working. Many people will join, but it requires slightly more effort still than many other social places around the internet.