It would appear core dumps in /var/lib/systemd/coredump/ are included in snapshots. Given that: 1. they are not required for most users and in any case are unlikely to be data of such value they require a backup. 2. can be extremely large and numerous 3. can be generated regularly e.g. systemd shutdown saga
it would seem rather undesirable to include them in snapshots, (if someone else could confirm this is not isolated to my system!) i would suggest removing cow from this directory.