JONV  Apple's guidelines say, "The user can toggle between the standard state and the user state by clicking the zoom button." So it's not a maximize button as many people think (a la Windows). And the "user state" is not defined by the guidelines; that's up to the developer. Since many users probably like the mini controller as their preferred way to control iTunes, it's perfectly reasonable and consistent to have the zoom button alternate between that state and a larger window.  
(Version 9.0.1)