Big /var/log/journal?

I don't understand why the /var/log/journal/ folder is so big.

For example, by executing the command systemctl -f, i see the fill.If I click on an email on Thunderbird, it generates dozens of lines that I consider useless.

Currently, I have more than 1.5GB (du -h /var/log/journal/) generated in 1 day!

Is there a method to reduce this log considerably without stopping logging?