When you start building your project, its layout is scaled to the width of your screen. This feature is called Scale Layout and allows adapting projects to different screens automatically. To limit the width of scaling, go to your project settings, select Desktop viewer, and enter the desired numeric value (between 1024px and 8400px) in the Scale Layout property.
You can disable Scale Layout from Grid&Guides or project settings. In this case, the layout width will stay the same across different devices. If Scale Layout is disabled, make sure to keep all your widgets within the blue frame: widgets placed outside it may be not visible on small screens.
This setting applies only to the Desktop layout. In Firefox, Scale Layout won't be visible in Editor mode.