It is possible that the system has files in there that you can't see, or there is something on the device that is taking up storage that you don't want on it. Have you tried checking the device for viruses and malware?
In the end, you may want to back up your personal files and contacts to a google account or a computer and then "Hard Reset" the device. You will have to reinstall apps after, but this will wipe any/all stuff off of it and start it over fresh.