Comment by kccqzy

The Apple backup strategy is purposefully broken. I’m already paying for 50GB of iCloud and it often claims that it cannot backup my iPhone despite having multiple gigabytes free. It turns that that during the backup process it operates on a file level, so if you happen to have a large file it will require both copies of the file to fit within your storage limit before the backup can complete. And guess what, several third party apps I use store all their data in a single multi-gigabyte SQLite database that’s written to every day.

As for cached and downloadable data, I have long ago turned off backups for many apps where the data is stored on a server anyways. Backing up these apps never makes any sense.