When you see your zpool exploding, but the “used” size isn’t that much, you may take a look at your snaphosts.
First find the dataset with the that has the largest snapshot
zfs list -o name,usedbysnapshots | sort -r -k2
example output
tank/mails 56.3G tank/store 261M tank/docker 2.38M
In this case the tank/mails dataset should be looked at.
Snapshots can then be listed for that filesystem by using a command
# zfs list -t snapshot -r zfs list -t snapshot -r tank/mails