Why "Documents & Data" is consuming so much space in iPhone?
Well it is normal. You can refer to this: https://support.apple.com/en-us/HT201656

If storage amounts on an iOS device differ from what you see reported in iTunes


  Since iTunes categorizes cached files as Other, reported usage for
  Music or Videos might differ. To view usage on your iOS device, go to
  Settings > General > Usage > Manage Storage.