@PeteM the size of the container is set based on viewport width:
As you view your site on larger or smaller screens the content inside will adjust to the available space based on the width of the viewport. So when you preview your site in the designer the width is constricted by the Webflow editor windows.
You can see here the width of each of your grid cells is set to be fractions of the grid container:
So as the container gets wider the fractional widths of the cells grow with it. You could set a max-width on your container that changes at different viewport sizes, but I would probably recommend thinking about this slightly differently, one of the huge advantages of grid is how it adapts responsively to different screen sizes.
Check out this example that Webflow built that contains a max-width for larger screens, but also adapts nicely on smaller screen sizes: https://webflow.com/website/newspaper-grid