Slim down an ios backup file
iMazing works with iOS 11 and allows you to slim down encrypted backups. You 


  With iMazing, your backups become fully editable, even if they are
  encrypted.





  Advanced users enjoy a complete toolbox for browsing and editing
  backups.
  
  
  Browse the file system of iTunes and iMazing backups
  Extract any file or folder
  Edit backup files, or add files to backups
  Encrypted backups are fully supported (password required!)
  


I have used iMazing in the past and for full and partial restores of iPhones and iPads.