My laptop's disk space was running low, and one of the large directories the Disk Usage Analyzer showed me was /var/log/journal/3a53c17cd7e5413c89e602ca479f5c6d/. It is the storage of the systemd journal.
You can configure systemd-journald to keep them below a certain size by setting SystemMaxUse in /etc/systemd/journald.conf to e.g. 100M.
Restarting the journal daemon with systemctl restart systemd-journald already causes older entries to be removed.