Over time, the temporary directory in cPanel server can grow big, even bigger than the size of your MySQL databases. It can be worse if you keep changing CMS. They will create more temporary files and make that /tmp folder big. I usually have this problem in my development server.
I consulted my hosting provider on how to clean up /tmp directory manually and safely.
They said that it is okay to delete all files inside /tmp directory except the folders and its contents.
Deleting the whole /tmp folder is NOT okay
Deleting the /tmp/sess_123456789123456789 file is okay
Deleting the /tmp/awstats/ folder is NOT okay
Deleting the /tmp/awstats/ssl/ folder is NOT okay
Deleting the /tmp/awstats/awstats.example.com.conf file is NOT okay
I have done that since 2014 every time I feel the /tmp folder is already too big. Nothing goes wrong so far.
I hope it can help confirming what is okay to do when cleaning up /tmp folder manually.