Whenever you download the data will first pass through the File Cache on its way to your physical disk. So as the download proceeds the file cache will grow up to a max where all of your RAM is being used. When the download completes the data in the File Cache will remain until say Applications need more RAM and then the File Cache will reduce. It's all about the system making efficient use of your RAM capacity. It's a good design.
3/20/14 at 2:19pm