I've had a small mastodon instance for over a year and recently decided to get to the bottom of why the "Explore" tab hardly ever recommended any posts.
When I was a programmer I never used ruby, but reading through the code I think it's down to a hard threshold of 5 minimum interactions for something to even be considered for the trending algorithm. This makes it hard to meet theses conditions on instances with a small number of active users.
It doesn’t store media forever that is routinely cleaned and configurable in the administration settings for the instance. The content cache, which is database storage used for the text and meta-data of posts is more typically an issue over time because this requires commands to be run on the server to free up space, which many people do not do. https://fedihost.co/blog/slug/managing-mastodon-storage