This is the only one I know of at the moment, though I'm still relatively new as well so there might be a better link for a list of instances and their sizes. Based on some posts I've read here, lemmy.ml has the most users, with lemmy.world at about half of lemmy.ml's and beehaw.org at about a third.

Also, yeah, I think we're on the same boat when it comes to why different communities are spread out. From what I've observed, "duplicate" communities in separate instances seem to all have their own "flavor" of that particular community. Taking the meme communities as an example, sh.itjust.works' memes have a bit of a French-Canadian flare to them, lemmygrad.ml's memes are made with a hammer and sickle, lemmy.world has more of that "general everyday memes" feel, etc.

On Reddit, it used to be r/memes -> r/memes, but here it seems like it's the opposite, like c/ -> c/memes. It's like having pizza in New York vs Italy -- they're both pizza but each country has its own twist to it.

I subbed to your community btw as I love me some Paramore myself. Hope to see everything work out easily for your community!