I'd say it's the cache like many others said. My mom's iPad is jailbroken and just earlier today, I used iCleaner and analyzed the "Applications" area and the cache files added up to 1GB! Yes, all those pictures from all your apps add up to a lot over time.
Whether it be facebook, instagram, tumblr, dropbox, amazon, ebay, and so on. iOS doesn't seem to delete those caches after some time, it just accumulates unless you delete the application and reinstall
Whether it be facebook, instagram, tumblr, dropbox, amazon, ebay, and so on. iOS doesn't seem to delete those caches after some time, it just accumulates unless you delete the application and reinstall