On a Mac, the green (+) button is not the maximize button; it is known as the zoom button, and it toggles between a "standard state" and whatever size and position the user has manually set for the window.
Apple's human interface guidelines say "Don’t assume that the standard state should be as large as possible; some monitors are much larger than the useful size for a window. Choose a standard state that is best suited for working on the type of document your application creates and that shows as much of the document’s contents as possible."
For someone with a 30-inch monitor, filling the screen with one window would usually not be desirable, which is why maximize is not part of Apple's user-interface design. It is up to the designer of the application to decide what the most useful size is for a window. Often, that will be the width of a printed page.
Of course, while some people like this behavior, others would like it to work more like Windows does. There are third-party utilities, like Right Zoom, that can be used to change the behavior of the zoom button.