I’ve been conflicted about this. For example beehaw has a larger technology community but I’m a fan of lemmy.world so I try to post interesting content there to help the site.
Beehaw might not be the best example since it’s defederated but you get the idea
You feed the community you want to see grow. If I was concerned about community size, I’d still be on reddit. You could post to the Beehaw community, which may mean more views, or you could contribute to your local instance’s community, which will show people that it is active, which will lead to people posting there as well, which will lead to growth. Or you could always post to both.
What if Lemmy had something like multireddits. So when you have several communities talking about the same thing, you can get a merged feed.
I think instances will eventually develop into hosts for specific topics, but that’s not a bad thing. You can still startup a new community on any instance. I think smaller communities will be spread over all instances.
This is not the worst idea I have heard. I think it would be really cool if people could get local instances going for their cities. Then have the communities be based on topics relating to said city.