When you view logs in ZS you can see a “configure” link. From there you can set thresholds to auto-compress and auto-delete logs. Perhaps these got turned off?
You could try to export your profile twice: Once normally, and once without logs, and then delete the profile. You could then restore the without-log profile, which should restore your ZS configuration without the logs.
Do you use a web proxy add-on or any web caching, or for that matter anything third-party that could take up disk space?