Windows 10 was originally designed with the intention of being compatible with Tablets and Smartphones. Naturally, the start screen in tablets and smartphones cover the full screen. It is not always the case when it is displayed on a PC and laptops with a big screen.
- Click Start -> Settings -> Personalization
- Click Start option on the left side of the window
- Turn on the option Use Start full screen
Next time, when you open the Start window, it will be in full screen. You can now start adding more tiles to fill the screen with commonly used Apps and images.