How large is the Photos library in iCloud? The size of the library in iCloud will determine, how small an optimized library can be.
Even an optimized library will need additional storage for working copies. And this additional storage is larger on the Mac than on an iPhone, because the thumbnails and previews for browsing will be larger on a Mac with a large display than on an iPhone with a small display.
On my Mac these working copies require roughly 25% of the size of the library in iCloud.
You can see which items are using the additional storage, when you ctrl-click the Photos Library and use "Show Package Contents". Display the folders inside in list view and compare the sizes. The "optimize Storage" can make the folder originals small, but all other orders will be needed,
For example - the sizes of the folders of my iCloud Photoslibrary - it is not optimized.

- originals: it contains the original image files
- resources: working copies with thumbnails, faces, previews for the media browser, etc.
- database: the results of editing, albums, folders - the structure of the library
- private: the sandbox for communicating with the background processes and external editors. It can be very large
- scopes: here you can find the local copies if the photos in shared albums
- internal, external: some system data and sandboxes for system processes
Be very careful not to modify anything. Just look at the sizes. Right after a new download from iCloud the background processes may need plenty of additional storage to cache data, while they are scanning and preparing the library. Give the Mac time to finish the initial scan.