By default, the Windows 10 Start menu will open up in a size that accommodates all of the pinned items. When the full-screen mode is enabled, the Start screen will overlay the entire desktop and provide a more generous amount of space to pin applications, and your live tiles will be larger. To disable the ‘Use Start full screen’ setting, click on the toggle to enable the feature, as shown below. You can close the Start menu at any time by pressing the Escape key on your keyboard or by clicking on another program or screen.
Source: https://www.bleepingcomputer.com/tutorials/how-to-make-the-start-menu-full-screen-in-windows-10/