Having more iCloud space than actual disk space (on macOS)? I'm thinking of buying a 128GB MacBook Air. I currently have 200GB on iCloud.

Is it possible to use the 200GB on my Mac via the "Optimize Storage" option ?

Let me give you an example: having 200GB of files on your Mac (as references), half of them only on the cloud and the other half actually on disk.

Or is the total 128GB count done even if the files are only present on the iCloud?
You can do this for iOS and macOS. The iCloud controls will tell you that there’s not enough space to download all the files and that you’ll have everything in the cloud, but only some files cached locally. 

So, you can enable save space / optimize photos which will resample images with lossy compression or just store thumbnails locally. For actual files, you will download them from the cloud when needed and only have a subset of them in local storage. 

You don’t need to have as much local storage as your full files stored in iCloud.