Possible these are files that are just not getting cleaned up.
It seems there is a known issue that was propagated from earlier releases that an "upgrade" doesn't exactly fix (i.e. the issue carries over with an upgrade).
Suggest uninstalling the client, remove /var/log/checkpoint and /var/lib/checkpoint folders:
sudo rm -rf /var/log/checkpoint
sudo rm -rf /var/lib/checkpoint
Reinstall 1.18.16 (or above) again
Monitor the size of the /var/lib/checkpoint folder again.
The issue was found in older client versions and was found to be carried forward in upgrade scenarios, so complete removal of the client, and clean up of the files is required to clear up the issue.
If you're still having an issue, might require TAC to investigate.