The 64GB Surface Pro Ships With Just 23GB of Usable Space
maximumpc.com
maximumpc.com
I think it would be pretty easy to account for why so much space is being used, but I still think that Microsoft is playing a little loosy-goosy with the amount of space that it uses just for the OS. Winsxs is always a source of confusion because it can use up to 15+ GB on its own.