Think of the iCloud library as one library that exist in the cloud. Setting aside the moment between when you turn it on and when all of the photos are uploaded for just a moment, each of your devices will have some mail of every image in the library as well as a larger version of the file.
On a device per device basis you can request the original sized file or let the system shrink, compress, optimize when storage runs low.
This design works well for people that just want to have one library and not several versions of the library. The point of iCloud photo library is if you have a 50 GB library total, you'll still end up with the same number of photos on each device even if the entire library size is 10 GB on an iPhone 30 GB on an iPad and 20 GB on your Mac.
The compression only kicks in once your space gets low on a specific device. I believe there are several settings of compression so that as you begin to fill up your storage with things other than the photo library, the images can continue to shrink up to the point where you have nothing left but thumbnails, presumably.