So the way Lemmy works is that a instance will only know about (and have the content of) a community if a user on that instance is subscribed to it. So when you browse All or search, only those communities that someone on the instance is subscribed to are included in the results. On a smaller instance that’s naturally going to be fewer communities.

Now if you search for a specific community by its URL that the instance doesn’t yet know about, it will actually go and fetch it for the first time. What this looks like to the user though is that the search shows no results, then suddenly 5-10 seconds later the results change and the community appears. Which is not a great UX for someone new. So again on an instance with more people it’s a lot more likely that someone else has already searched for and subscribed to what you’re looking for so that you don’t see that issue