- Make sure Global is highlighted in the left sidebar.
- Click the Configure New Trigger button.
- From the Trigger drop down select Leftclick Green Window Button
- From the Predefined Action drop down select Zoom Window Below Cursor (this can be found under the Window Interaction section).
Bring back the old window zoom button in Yosemite
Yosemite changed the default behavior of the window zoom button (green circle) to bring the window full screen instead of resizing (zooming) to the content. You can of course hold down the option key on your keyboard when clicking the button to toggle between full screen and zoom. However, if you want to change the green button to always zoom again and not have to hold down option, you can accomplish this with a free app called BetterTouchTool.
Once you have BetterTouchTool installed, go into Preferences and configure a new trigger.