That space on iCloud and the space on your phone are 2 different things. And before you ask, no you can't add more memory to your phone.
Do this instead to clear out some space on your phone.
On iCloud, create a folder called "Archived Photos". Upload them (your photos) to there. Double check to see if they all uploaded there. Then you'll be able to delete them off your phone. Even though I use a 128GB iPhone 6S Plus and a 256GB iPhone 8 Plus I will never have this issue, it's there just in case.
As an alternative, you can have the phone store smaller resolution versions in your phone, while iCloud will have the original photos.