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?
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
I'm not sure how Boost implements it, but two different ways it might go
As for the subscribed numbers, that is correct.
You can see this in depth discussion here and compare: https://lemmy.ca/post/5674368
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