I think you are correct now; a number of fediverse projects actively seek to avoid getting big. For example pixelfed creator has closed signups on his instance

I think you are correct now; a number of fediverse projects actively seek to avoid getting big. For example pixelfed creator has closed signups on his instance and mastodon encourages people to sign up for different instances (and I a number of existing mastodon instances has also closed signups because they are big enough, and everyone except banned users and instances can just federate anyways.)