The phone will most likely not reduce the storage space used by photos until it needs the space for something else. If you want to manually do this you can. Make sure all photos are synced to the cloud, turn iCloud Photo Library off, delete all photos from you device and from the recently deleted then turn iCloud Photo Library back on with the option to optimize. What you will get from the cloud is the thumbnail size photos which take less space on your device. I have about 5000 photos and 300 videos and use optimize. On my device I'm using about 1.4G of space. In reality it takes about 26G of space on the server. Not bad.