Over time, temporary directory in server can growing bigger, 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 /tmp folder big.
That is what happened to 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 everything inside it.
I did that and the size of /tmp directory went to 1.11MB from 32.57MB. Nothing goes wrong so far.
Hope that can help confirming what is okay to do when clean up /tmp folder manually.