I found the size in /var/log/journal/ increases quickly.

One of my VPS has only 40GB disk, the journal takes 10GB.

After searching and I found this solution:

Ask Ubuntu: big /var/log/journal?

I followed the methods and made the problems resolved.

I only needed to edit the file /etc/systemd/journald.conf and put this line in it:

 SystemMaxUse=100M

Then restarted the journald daemon:

 service systemd-journald restart

After doing so the dir get cleaned automatically.