Standards for page width and height allow pages to expand both horizontally and vertically; this caters to large screens, while also maintaining the visual appearance of the page at minimum sizes.
By default, Web pages do not include content in the main application content pages that expand horizontally when you resize the window: only the header and footer items that are right aligned move when you resize the window. Typically, all content is left aligned. To center the main content, rework the base page structures and styles.