hello @forbin, i had some deja-vu as restic+zfs was discussed with you before on similar topic of data sizes. 
Refer to Why is the repo 300% larger than the source folder? - #23 by forbin
-
Did that ever get resolved?
-
How do you conclude that the remote disk now ran out of space; based on the du command or did the zfs system notify that it is filled up?