The big "unless" is not buried, it's common sense. If you start storing tons of information on iCloud Drive, do you think that your local drive is going to magically expand to keep all of that data? Possibly if you would read the literature on iCloud Drive you would get a better understanding on how it works and what it is intended to do:
The same is true of iCloud Photo Library. What can be kept in full resolution on your iOS device or computer is kept there. However, if you are storing a lot more photos than can be stored on your device, the iOS dynamically reduces the number of full-resolution photos and replaces them with thumbnails that need to be downloaded.
The same is true of the contents of your iCloud Drive. The Optimization process dynamically puts older documents back into iCloud Drive only, and leaves the ones you work with regularly on your hard drive. You have the option, at any time, to download any document that is only in iCloud Drive so that you can work with it offline.
So, if you want to have the sharing capabilities of iCloud Drive, then you need to understand that it does not expand your Mac's memory. It is a virtual drive that houses documents and data, and to the degree that it can keep those documents and that data on your Mac, it does. But if you exceed the capabilities for keeping all of that information on your limited memory Mac, it holds it in the cloud for you until you select to download it for editing purposes.
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