Difference between file size vs. "size on disk" in Windows 7 / Vista / XP
You may have come across the case where you look at a file property to get its size (or get the size of multiple files and folders), and noticed that Windows 7 / Vista / XP was showing two different sizes: "Size", and "Size on Disk". So what is the difference?
File Size vs. Size on Disk
Simple definition of file size: Your files are containers of information; the type of information stored inside a file determines the file's size: a plain text document in Notepad is typically small, but the exact same text inside Microsoft Word is much larger, because Word includes a lot of "metadata", like text formatting, colors, bullets, etc. inside its file as well. A picture is even more tedious to describe for a computer, and it needs to write information about the position and color of every single dot, or "pixel" that makes up the photo or image (in addition to GPS coordinates, the name and model of the camera that shot the image, etc.)
Simple definition of size on disk: unlike size, which is an absolute measure of the amount of information a file contains, the size on disk depends from one computer to another, and from one moment in time to another on the same computer. Why? Because Windows 7 / Vista / XP do not store files as a single piece of something: instead, they have to store a single file in multiple sectors on the same disk - and another piece (or more) of the disk (hard drive) has to be used to keep track of all the pieces that make out the entire file!
Note: this is the reason why a well and regularly defragmented hard drive is important, and makes your computer run faster. When all the free sectors on your disk are close together (new hard drive, or a defragmented one), all files are spread in sectors close to one another - so your PC has to do less searches to find all the pieces of a file.
Bottom line on difference between file size vs. size on disk: if you are concerned about the amount of space a file will take on your hard drive, you should look at the "Size on Disk" indicator; if you want to figure out if a file is small enough to be emailed or uploaded, look at the "Size".up ↑