I’m going to guess that there’s some kind of pruning going on just by virtue of the size of them. My Mastodon instance which is pretty well unused other than the few dozen people I followed on there has an image store or around 300 GB last I looked. Lemmy seems just as hungry though I’ve ran it for less time. Scaling that up to the biggest instance around and I’m sure they’re at many multi TBs of storage just for things to chill there. Those on outside instances might have cached copies that the home doesn’t though.