You are probably limited by the GUI.
You could go here : Status > System Logs > Settings
and change
c5dad0b9-bcbc-4136-8867-484d78c846fc-image.png
to something a bit bigger.
Check also this :
4280c79f-7433-4ea4-b1c1-6f317c100f08-image.png
where you can set overall log file size.
If you have many G bytes to spare, you can make these files a bit bigger.
On very small devices : be carefull.
But you can also apply the "IT" way : you don't care about GUI ... go native access right away. Go to the source.
Use the console, or, like everybody else, use the SSH access, and look her /var/log/ as that is the place where logs are stored on nearly every "computer" on planet earth.
You'll find the system.log file.
Btw : typically, I have 20-30 lines a day in the System log file.
So "only the last 5 minutes worth" is pretty strange : what is happening in there that your pfSense logs that much ? ?
Massive logs == normally : an indication something not-ok is going on.