journalctl uses a lot of space on my disk

The aswer is you can check the used size and you can shrink the used space.

journalctl --disk-usage
journalctl --vacuum-size=1G

Other Options are

--vacuum-size=BYTES Reduce disk usage below specified size
--vacuum-files=INT Leave only the specified number of journal files
--vacuum-time=TIME Remove journal files older than specified time