If I store my photos in iCloud why do they take so much space on my phone and my Mac?

iCloud acts as a synchronizing service between all your equipment, it is not external storage. The idea is you take a photo, it is copied to iCloud, then copied to your computer. So it is in all places.

https://discussions.apple.com/message/33289584#message33289584 - "iCloud Photo Library is a syncing service. Any photo you delete from a device where iCloud Photo Library is turned on will also be deleted from iCloud and from any other device connected to that iCloud Photo Library. You can potentially reduce the storage space required on a device by turning on Optimize Storage. See: iCloud Photo Library - Apple Support - https://support.apple.com/HT204264 And how to turn off optimization: https://discussions.apple.com/message/33340829#message33340829