I reset the Corpus. I wanted to also trim the log file. It has entries going back to 2014. This was something I thought the prune data would handle, but apparently not.
The Prune full message data in log option removes the raw message source data, which is what uses the most disk space. More options for automatically pruning the log metadata are planned for a future version of SpamSieve.
If you need to fully remove the log entries now, you can select them (up to 1,000 at a time) and press the Delete key or quit SpamSieve and then remove whole month folders from the /Users/<username>/Library/Application Support/SpamSieve/Log.spamsieve-logd/ folder. For example, to remove every log entry from February you would delete the Log 2024-02.spamsieve-log folder.