I just tried out the shared album thing. I shared a photo whose original size is 3.4MB. The photo you get when you download it from the shared album on iCloud is 925KB. And, I'm pretty sure there is no checkbox you can check to prevent that ️. I love lots about the apple ecosystem, but their photo management/sharing system is not one of them. It boggles my mind, as lots of Apple engineers have families. But, despite that, they don't have any system for seamlessly sharing photos (or entire libraries) with family members.