You can also double click the title bar to expand to fill the screen, and double click it again to return to the previous size. One caveat to this is if your application saves window sizes on close and you close with a full screen window and re-open, it won’t shrink again because the “original size” was full screen when the window was opened
Setting the "Double-click a window's title bar" action to Zoom just fills the vertical space. It doesn't adjust the width to fill available space, which is usually what I want. Finder, as an example, almost never sets the width to something usable.
True, it would be more accurate to say that what "zoom" is designed to do is expand the window as much as possible to display all the content, however the application defines that. For many applications, the result is a fully maximized window. On my MBA, Notes, Messages, Calendar, iTerm, Solvspace, and Mail all just fill the screen. For other applications that report specific content area to the OS that can mean something different. A finder window generally reshapes to show all the icons in icon view, or to maximize columns and names in list view.
The most annoying one is Safari, which arbitrarily decides on what the "best" window width is, even if the website could easily reflow to fit a wider window.
