Well, I was pretty sure. Check the last entries on the list.
These are cached files from plasma themes you have tried/customized in the past.
Each one of them takes around 85MB in your disk.
There have been some bug reports about kdecache filling up when the user is playing around with themes and icons:http://forum.kde.org/viewtopic.php?f=22&t=96677
You can safely remove the folder /tmp/kdecache-arnold.
Although I have deleted it on-the-fly with no apparent issues, it is suggested that you do that without KDE running.Do-it-at-own-risk way:sucd /tmprm -rf kdecache-arnoldSafe way:1.
Log out of your current KDE session.2.
At the login screen press Ctrl+Alt+F1 to go to tty13.
Type su and enter your root password.4.
rm -rf kdecache-arnold7.
When you log back in, check your disk space. You should have around 2.8GB more free space now.