I wanted to free up some space on my iPad, so I looked at Usage, and it said Mail was taking up the most bytes, so I went to my All Mail and deleted all the adds (which was a lot). Then I went to trash and deleted everything in there. Then, I check Usage again. It had said at first mail was taking up 19.4 MB, then AFTER I deleted a ton of things, it went UP. Now, it says it's taking up 33.4!! It did the exact opposite. I deleted a bunch of stuff, and the amount of storage the app was using went up... what's happening?? Thanks!
First, try signing out/disabling all Mail accounts. If that doesn't work, keep reading.
iOS apps created by Apple create and store logs, especially Mail. By deleting all the stores emails, you probably created 100's of log data about the deletion. You need to delete all of these.
The only way to do that is to restore your iPad. First, plug in you iPad to your computer and open iTunes. If "Other" takes over a gigabyte, you will need to restore. If not, disregard everything after this.
To restore, first click "Backup". After the backup is done, hit restore. It will download the latest iOS, then restore. After it is down restoring, set it up like it was new until it asks you to restore from backup. Select the backup you made earlier and let it work its magic.