iCloud backup size is significantly smaller than photo library itself
I noticed that the backup size for my iPhone is significantly smaller than the size of just the photo library itself, and am wondering why? I have iCloud Photo Library turned OFF, so this is not explained by the fact that with that feature turned on the photos are excluded from the "iCloud backup". I suspect there may be some compression going on, but not this much. Any thoughts?
iPhone 6s Plus, iOS 11.4