iPhone mail using 700Mb with no accounts
It seems like I have answered a number of questions with this particular solution, but let's go for it again.

Often times a backup, wipe, restore process will clear out the cruft still sitting around in the rather opaque IOS filesystem. I don't know why but it works:


Backup: Connect the iPhone to your computer and do a manual backup to that computer. Password protect the backup so you don't have to re-enter passwords.
Wipe: Once you have a good backup go into the iPhone > Settings > General > reset > Erase all content and settings. And let it do its thing.
Restore: Once the phone reboots and is waiting on it's welcome screen plug it back into your computer and restore it from the backup you just made.


9 times out of 10 all the inexplicable space usage on the phone goes away and you have got your space back.