If you have upgraded your Mac OS X to Yosemite, you will find that there are plenty of changes to the user interface. One of them is the behavior of the green maximize button on every window. In previous versions of OS X, clicking the green button will maximize the window to fit the document width. In Yosemite, clicking the green button will now send the window to full screen mode, and new users will panic as there doesn’t seem to be any visible way to undo it.
When you hover your mouse above the green button, you will see that it turns into a “full screen” icon instead of the usual maximize icon.
If you simply want to maximize the window the same way as in previous versions of OS X, here’s the trick: press “Option” while clicking the green button.
In fact, when you press “Option” and hover your mouse above the green button, you will notice that it shows a “+” maximize icon instead of the usual “full screen” icon.