Computer backup of iPhone incorrectly includes iCloud photos

Hi, could you help me with this? I want to use iCloud Photos but this proves unfeasible because Finder backups of iOS devices includes a copy of iCloud Photos (contrary to what Apple documentation states). Thank you!

- Description - 

I have an existing iCloud Photos library that I want to sync on my devices (Mac, iPhone, iPad). The Mac needs to download and store originals in the macOS system photo library, so as to enable long-term Time Machine backup. iPhone and iPad will ‘Optimize Storage’. As I routinely backup iPhone and iPad via Finder, I want to make sure these device backups don’t include a copy of iCloud Photos as these would end up as triplicates on the Mac, therefore wasting expensive internal SSD storage.

But the reality is, contrary to what Apple documentation states, these device backups do inflate considerably when iCloud Photos are enabled, as iCloud Photos are seemingly included and overconsume Mac storage for every iOS device that is backed up. Unless this is fixed, this will mean iCloud Photos consequences are just so impractical and costly that this service must be avoided.

- Steps to reproduce -

1. Reset iPhone’s photos to a clean slate: disable iCloud Photos on iPhone, make sure photos sync via Finder is disabled too, and delete all remaining photos from iPhone (simplest via Image Capture macOS utility).

2. Backup iPhone via Finder (backups are encrypted with password). Note down the size of the backup as can be found in ~/Library/Application Support/MobileSync/Backup - this should be the base device backup size (without photos).

3. To make sure Finder is ‘aware’ that backups are to be optimized, temporarily enable synchronization of Photos via Finder, selecting photos only from ‘Last Week’ to expedite the process. Backup the iPhone via Finder and confirm that the backup size hasn’t significantly changed and is still at its lowest.

4. Re-enable iCloud Photos on iPhone (accept the warning that this will override Finder sync, and that all photos will effectively be deleted). Let the iCloud sync accomplish. Current status is as intended: iCloud Photos enabled on Mac (with Download and Keep Originals), and also enabled on iPhone (with Optimize iPhone Storage).

5. After a few days when the iPhone had a chance to sync iCloud photos (plugged into power), perform another backup via Finder. Note down the size of this new backup.

- Expected results -

As stated in https://support.apple.com/en-us/HT204136 a macOS backup of iPhone (that is performed via Finder or previously iTunes) should NOT include « data already stored in iCloud, like iCloud Photos ». The size of the iOS backup should not exhibit any significant growth.

- Actual results -

The issue is confirmed on the same Mac (macOS 11.0.1) with 2 different iPhones (iOS 14.2) for 2 distinct Mac users and 2 distinct iCloud Photos library:

For iPhone user #1 with 16 GB iCloud Photos library (5100 photos & videos): Finder backups went from 3 GB / 11500 items (clean slate) to 8 GB / 23000 items (after iCloud Photos were enabled), the 5 GB increase in backup size equal to ~30% of the Photos library and 2.2 additional items per iCloud photo.

For iPhone user #2 with 88 GB iCloud Photos library (33300 photos & videos): Finder backups went from 13.8 GB / 34000 items (clean slate) to 43.6 GB / 98000 items (after iCloud Photos were enabled), the 30 GB increase in backup size equal to ~33% of the Photos library and 1.9 additional items per iCloud photo.

Overall, Finder backups of iOS devices are wasting 35 GB of Mac storage due to iCloud Photos, which would probably double to 70 GB were we to activate iCloud Photos also on both iPads (which was initially the intent and is proving unfeasible).