Window Zooming in OS X does not magnify a window's contents, but rather resizes the window to fit its contents. While this is the most common use of this button, the function is application-specific and may do anything from changing the overall window appearance such as switching to the mini player in iTunes to doing absolutely nothing at all.
Invoking the zoom function is done by clicking the green button to the right of the close and minimize buttons, but it can also be accessed from the Window menu that most OS X applications have. Unlike the Close and Minimize window buttons, the Zoom button does not have a dedicated keyboard shortcut, and some people may wish to activate it by shortcut.
To do this, you will need to take advantage of the fact that the Zoom function appears in the menus for an application, and assign a custom keyboard shortcut to this function.
- Go to the Keyboard system preferences.
- Go to the Keyboard Shortcuts tab.
- Click the plus button to add a new shortcut.
- Ensure that All Applications is selected from the drop-down list.
- Type "Zoom" in the Menu Title field, and assign a unique key combinaton to it.
- Click Add to save the new shortcut.
Suggestion: I am a fan of using Control-Option-Command plus another key of choice for custom keyboard shortcuts. These three modifier keys are rarely used together in shortcuts, and they're easy to press at once since they're right next to each other.
When this is done, as long as there is a Zoom option for a specific application then when this application is in focus, pressing the specified keyboard shortcut should activate the zoom feature. This can make resizing windows to fit their contents much more convenient.