I wonder how much disk space the big instances are using... I'm only at a dozen users. I have subscribed to most of the largest communities across the "threadiverse" so my instance is caching a copy of everything happening across those. I'm under 2GB space used so far.
I imagine it's mostly the media uploads. I think that doesn't get cached, just the text. But anything a user uploads will get saved to their instance's storage. I think.