If you are one of the ios6 users who's iphone had a treatment of apple's storage vampirism AKA ios7 OTA which is automatically downloaded to your phone over WIFI please refer to the following thread I created which will show you how you can reclaim that lost space and prevent future downloads over your WIFI.
https://forums.macrumors.com/threads/1646519/
https://forums.macrumors.com/threads/1646519/