How many photos are you having in total? How much storage are they using in iCloud?
You should expect your photos to be using some local storage on your iPhone, even if they are stored in iCloud Photos. Photos will keep smaller, optimized versions for browsing on your iPhone, so you can select and download them, when you want to view them. If all photos have been uploaded to iCloud, they will still need local on your iPhone - as a rule of thump at least 10% to 20% of what the photos and videos are needing in iCloud. Your photos may already have been uploaded completely to iCloud, if the status message is saying "syncing". If there were photos that still need uploading, you would be seeing "Uploading" in the status bar.
A photos library of 400GB in iCloud would need about 40GB on your iPhone. If you cannot afford that much storage for photos locally on the iPhone, try to keep less photos in iCloud - save the photos you do not need accessible on your device on external drives on a computer.