iCloud Drive "steals" space from my Mac Mini
I have the new Mac Mini with 1 TB HD, 200GB in iCloud Drive and an external 2TB HD. Whenever I copy files to iCloud Drive, Finder indicates that the space available on the Mini decreases at the same rate, as if I'm duplicating the files in the Mini itself. For example, I currently have 650GB available in Mini, if I copy 10GB of files (photos, films, documents etc) to iCloud Drive, the free space on the Mini will drop to 640GB. The same thing happens with Google Drive, but that doesn't happen when I copy files to the external HD. Why is that? Am I doing anything wrong? It is a "bug" of Finder or OSX?
Mac mini, OS X Yosemite (10.10.1)