Photos will use the available storage on your MacBook Pro and keep downloaded photos stored locally, even with "optimise storage" enabled. It will remove the some of the originals you have bot used in a long time, if the storage gets low. This is usually very useful, as you will not be wasting network capacity by downloading the same files over and over again. Am optimised library will always try to keep a smaller working set of recently downloaded files at the full resolution.
But if you really need to free a lot of storage at once, you can force the optimisation on your Mac:
- Quit Photos, if it running.
- Select your Photos Library in the Finder. Simply delete the complete Photos Library (provided, you are sure that all your photos have really been already uploaded to iCloud). If you are not sure about this, keep a copy of the library on an external volume.
- Create a new empty library and make it your System Photos Library and iCloud Photos Library, with optimise Storage enabled. Initially the library will be very small and fully optimised. It should suffice to install a system upgrade.
See: How to force Photos for Mac to Optimise the Storage Immediately
On an iPhone or iPad we cannot delete the Photos Library. If we need to get rid of the downloaded versions on an iPad or iPhone, we have to sign out of iCloud Photos. Make sure that iCloud Photos is really disabled, then delete all items from your Photos Library. and empty Recently Deleted as well. Enable iCloud Photo again, enable "Optimise iPhone Storage, let the syncing start again. Initially you will not have many downloaded photos in your library.
