Starting in Mac OS 8 the way reporting application memory useage was changed. The information in the "About This Computer..." window now reports the application's memory partition (Preferred Size setting) and the amount of memory the program is using as temporary memory.
Temporary memory, or main memory, is made of physical RAM or Virtual Memory, a combination of physical RAM and hard disk space. Temporary memory allows the Mac OS to keep portions of programs and data currently in use by the application nearby for quick access.
Some programs may use more temporary memory than others. Programs that do not need a significant amount of temporary memory may display a number in the "About This Computer..." window that is very close to their Preferred Size setting. In both instances, this is normal behavior.
The "About This Computer..." window may be used to determine how much memory is available for an application to use. Using the Largest Unused Block memory size as a guide, set the Preferred Memory setting in the application's "Get Info" window so that additional temporary memory can be allocated without causing an out of memory condition.
If your computer is running out of available memory after launching an application, increase the Virtual Memory setting in the Memory control panel. For more information on Virtual Memory, please see the following Knowledge Base article:
15854: "
Virtual Memory: How It Differs On Power Macintosh"