On Lemmy that is. Kbin I believe replicates everything (unless I set my server up wrong). My server at the moment pulls in around 1.5 GB a day it seems. There is a pull request open on the kbin git repo for a feature to auto-remove old media. Personally I'd like the ability to turn on/off media replication. If an instance wants a complete copy in case of defederation/disconnection somehow, they can opt in and mirror all media that comes in. Most servers should just link to the original image source on the originating instance though.