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. That’s it.
