My iphone is 32GB, it's an Iphone 5...
If you need more details please let me know.
Restore the phone as new (not from backup).
I witness this when you sync and unsync. What solved for me is that I eject the iPhone from iTunes and then plug it again. Others goes back to small number MB.
Do you have email configured. If so, these may be your downloaded emails.
Turn off automatic syncing in iTunes and then try a backup and restore. You can also customize your backup using iCloud. iCloud will not increase the storage capacity of your device.
Keep in mind that the operating system (iOS) takes up a chunk of this "other" storage, as do contacts, emails, text messages, apps, anything that makes your phone operate, etc.
Check out this discussion:
Retrieving data ...