There are a few things that can be done here. I'll try to be as concise as possible here to help:
Option 1. If your phone supports SD cards, I would recommend you attach your phone to a PC, clone your current SD and then duplicate it onto the new SD. This isn't really ideal as from your question it looks like you're not actually using the space.
Option 2. It may be beneficial to un-install several applications that are not frequently used and delete a few files. Once you have some spare room you can install applications like: Easy Uninstaller to remove applications and their leftovers, All In One Toolbox to help uninstall applications, delete temporary files and perform general maintenence of your phone and Well File Manager.
By applying the use of one of these you can uninstall redundant applications, clear temporary files and redundant files from applications. One a side note, there is an option in Settings to keep application data- you may wish to review this and see if it helps.
Option 3. Perform a backup of your phone, factory restore it and batch reinstall all applications. This can be achieved with the use of a tool called Super Backup Pro for Messages and Contacts. All applications can be recovered via the play store.
It would however be curious to see what files are being stored in the directory reporting the large use of space. Unlikely, but it may also be beneficial to perform a scan of your phone to ensure there is no malicious applications causing this odd occurrence.
If this answer is sufficient mark it so. If you feel as though I have not answered this correctly please provide more informaton.
