I'm new to this but I don't understand how you're meant to find the correct biggest community if it's on a different server than the one you're on.
If I search for it I only see communities on the same server as me. How do I know if a community just doesn't exist or if it's just on a different server?
Is there a way to search for the largest community across all servers?
We also have a guide on finding communities pinned on !communityPromo@lemmy.ca
As for your question:
I use lemmyverse.net to find a few communities. Usually one is actively used and the rest are dead. If a few have recent posts, I'll subscribe to them all to get the content. Later you can post to all of them, or the one you like more (friendly moderators, nicer discussions, etc.)
Unfortunately to see total subscriptions, you either need to go to the instance itself or wait for 0.19 to fix the issue where it shows the lower count (number from your instance only)
I'm not sure how Boost implements it, but two different ways it might go
Searching for communities on your instance (from the browser) will only show foreign communities that are (A) from instances your one is federated with and (B) subscribed to by someone on your instance. This is because the server just hasn't fetched the other communities (because it doesn't talk to defederated instances, and because there could be an infinite number of communities out there so it only loads the ones being used)
https://lemmyverse.net collects data on communities by itself, outside the instances, and so it is more likely to show you the big picture.
I also made a post the other day in this community to request that Boost show the correct subscriber count, but people pointed out that this is being changed in the next Lemmy version anyways: https://lemmy.ca/post/9717821
Much easier to sift through this site than the built in list. Also includes all communities from all instances, not just the ones already federated to your instance.