I just learned that you can get this information in the "Queue" screen, where it details the size of the content from the Disk, and the size of the content from the Network (what has to be downloaded over the internet). What has to be downloaded over the internet does indeed vary from title to title, but significant gains can be obtained by using physical media for those of us with relatively modest internet speeds. I just installed "The Witcher 3 - Complete Edition", and all the content came from the disk - and the install was completed in less than a half hour! To me this is a compelling reason (and happy news) to use physical disks!