CDs and disks are both circular in shape. They spin on an access, which allows data to be read off the disk at a rapid rate. Your files are saved on the disk in the order they are entered.
When you make a file bigger, part of the data resides where it was first saved and the remainder is saved after other data. In other words, part of our file would be located in one part of the disk and another part would be located in another part.