By default, the maximized attribute on af:document is set to true. Upon rendering content, any outer margin, border, and padding are removed, and the body is stretched to fill the entire browser window.
However, this does not mean that setting maximized to true is sufficient when you want your page contents to fill a browser window. When the user resizes the browser window, the af:document component does not reposition and resize its children components accordingly.
If you want a component to fill an area, such as the browser or a designated part of another component, then you must enable stretching for that component. If a stretchable component is a root component (shown at the first level of a component hierarchy in the Application Navigator), then it stretches automatically.