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, open the Grid & Layout settings and enter the desired numeric value (between 1024px and 8400px next to Limit scale at).
You can disable Scale Layout from your Grid & Layout settings. If disabled, layout width will differ across different screen sizes. When disabled, make sure to keep all your widgets within the viable area. Widgets placed outside the highlighted area may not be visible on small screens.
This setting applies only to the Desktop layout. In Firefox, Scale Layout won't be visible in Editor mode.