I have the same problem with a client. Backing up what I’d guess is a few tens of GBs, the cache still grows to three-digit GB sizes. If I delete the cache it’s recreated (you can do that too, deleting the cache between restic runs won’t hurt) but starts hogging disk space again, growing several GBs per week. I’m guessing it’s related to how much metadata and such are saved with each run, which I started trying to figure out in Tracking down large metadata changes .
Perhaps our symptoms have a common root cause.