Antgul, OS X will expand a window just enough to show all the contents on a page without empty space around it. For instance, if you are on a page that has one row of photos or something going across the top third of the window, clicking on the green Zoom button will only expand the window enough to show those photos without all the empty space below them. If you then navigate to another page that has a lot of text taking up the center portion of the page, clicking the zoom button will lengthen the window all the way down but not to either side. Then if you were to navigate to a large photo or a page that is filled from left to right and top to bottom with content, clicking the Zoom button will probably make the window "full screen".
This is apparently done so that no space is wasted, but if you really want to make the Zoom button behave like a Windows "Maximize" button, you can download a little tool called RightZoom.