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.
