[RightZoom](http://www.blazingtools.com/downloads.html) is what you want (scroll down to the bottom of the page or use [this MacUpdate link](http://www.macupdate.com/app/mac/30591/right-zoom/))

>This free tool fixes an inconvenient behavior of green Zoom button in Mac OS X. It will always expand windows on a full screen instead of strange resizing.