Huge iTunes Backup

I use iCloud Photo library.

On my iMac and iPad, I have set them to Download Originals. On my iPhone, I have Optimise Storage.

I like to backup my iPad and iPhone to iTunes as well as iCloud. My iPhone backup is a normal size: 3.57 GB. My iPad backup used to be that sort of size. However, since turning on iCloud Photo library, it has ballooned to 32.66 GB. As my Photo library size is about 28 GB, it seems that iTunes is backing up all the photos from my iPad! This is crazy. I don't need a duplicate of my photos on my iMac; they are there already in the Mac Photo library already as originals. I want to reduce my iPad backup to the same size as my iPhone.

I have changed my iPad setting to Optimise Storage and backed up to iTunes, but it is still the same size. My suspicion is that it may take a month to reduce. Even worse, it may never, as I still have 35 GB free on my iPad.

Any advice would be much appreciated. Perhaps my best bet is to switch off iCloud Photo Library completely on my iPad, then turn it on again with the Optimise Storage setting. With luck, it will then be much smaller. But I think this is a bug on Apple's part. iTunes shouldn’t be backing up the photos at all; they are all in iCloud.