When Windows 10 Insider Preview build 10122 was released, one noticeable change was the removal of the toggle to expand the Start menu. According to Microsoft, people usually pick the appropriate size (or just go with the default) for Start and stick with that, which makes having this button being present all the time unnecessary. If you want the full-screen Start experience on Windows 10 desktop mode, you have to enable it in the settings. We’ll show you how. It’s really simple. Watch the steps in our video.
Source: iMore