The area of each rectangle in a view window is proportional to the size of the file it represents. There is, however, more than one way to measure a file's size. GrandPerspective therefore supports two different file size measures:

• Logical size, which corresponds to the size of the information in the file. For example, the logical size of a simple ASCII text file is the number of characters (including spaces and linebreaks) it contains.
• Physical size, which represents the space that a file actually takes up on disk. A file's physical size can be the same as its logical size, but is typically slightly larger due to the way files are stored. For example, a file with a logical size of ten bytes may, depending on the file system, have a physical size of 1024 bytes.

By default, GrandPerspective reports logical file sizes. You can change the file size measure that is used by changing the application's Preferences.