I know the post is old, but i was dealing with this problem today and this popped out from my search engine. You have probably changed phones but to set the record straight for future searchers...
HERE IS THE SOLUTION:
1. text, mail, in between other apps may take a huge chunk of space even if, for example "messages" are deleted completely, the problem is they are not. Here comes the "Saved messages" part: that phrase means that they have been temporarily delete from your phone but they are still stored somewhere (It can correlate to PC-Trash bin situation, were you delete an icon but they go to Trash bin before permanently deletion) In the iphone case, they go to a type of invisible list where the system know they are up for deletion but does not have a "authorized permission" to permanently delete. To resolve this, connect your phone to iTunes and Sync your phone. That sync between your phone and computer is that "authorized permission" i was talking about to permanently delete your text and free up space, also deleting document and data space. Take it slow, erase about 20-25 messages and sync, to make the sync process easier and faster for the computer to process.
I had almost half of my phones memory in saved messages (3GB) and i kept restoring phone and erasing all data but nothing, until I went digging further and further in some forums and found a similar explanation of what i just wrote.
For apps, you may find temporary solution by deleting and reinstalling the app. Documents and data increase from apps probably comes from constant updates to that same app. If you delete it and reinstall it, it was installed with the current update, while the older app moved "old updates" into an invisible trashcan in order to make space for the new update, and that invisible trash can is what I like to call Documents and data
For photos, you may want to try using apps such as Dropbox, which stream photos to your dropbox account you can also access from your computer, storing those photos safely in your computer. You can then delete them on your phone and repeat the "Sync process" explained above.
Any doubts, you can just message me.
RJ