I keep getting a message that the storage on my iPhone 6s is full. I've optimized photos, deleted data from the most space-hungry apps and still get the same message. Then I noticed that the "system" app is using almost 13 gb of my 16 gb of storage, and each time I delete an app or create space in some other way, the amount of storage used by the "system" app grows to take up that space. What's going on here, and how can I reverse this problem (or at least avoid being eaten alive by the system monster)?