Memory Query

Welcome to the Apple Community.

That's optimisation for you.

In order to save space on your mobile devices (and indeed your Mac if you wish to do so) turn on optimisation for photos on these devices (Settings → Photos → Optimise Storage on your mobile device and Photo Preferences → iCloud → Optimise Storage on your Mac)

Optimisation is dynamic, it reduces the resolution of your photos as and when needed, so you may not notice it occurring at first. However, when it is needed, it is very effective and users have reported libraries being optimised to much less than 10% of their original size in some cases. My own library is about 9% of its original size on my phone.

Yes, I know it's switched on, that's why the photos are taking up much less room on the phone. Why would you like to reduce the amount of space used in the cloud?