Mac OS X Snow Leopard: The Missing Manual (Missing Manuals)

D.51. Maximize button

On the Mac, clicking the zoom button (the green button at the upper-left corner of a window) does something like the maximize button in Windows: it makes your window larger. On the Mac, however, clicking the zoom button never makes the window expand to fill the entire screen. Instead, the window grows ”or shrinks ”precisely enough to enclose its contents.