It’s not the image area of the screen, but of that container on your site.
If you inspect the page with a web inspector like Chrome DevTools or Firebug, you’ll see the size of everything and how it’s put together.
The slideshow is in a container called “rt-container”, that is 1200px wide, and there’s a little padding or margin in there that makes the slideshow space 1196px wide. So no matter how large you set the slideshow, it won’t go wider than the width of your site, which is 1200px. The rest is background, not content.
You could change the width of that container, and re-update the slideshow widths if you want it wider, but Meteor Slides needs a fixed height and width to work. The slideshows are responsive and scale down but they have a max width, they won’t go full width or full screen.
Generally if I want a “full width” slideshow, I’ll plan for it to fill the screen up to a certain point, like maybe I want the slideshow to be 1600×400, so that on screens at 1600px or narrower, it is full width, but then above that on wider screens the slideshow is centered on the page.
That might not be what you are going for, but I think it’s important to have a max size, otherwise the quality starts to suffer, you either need very large images that slow your site down, or you are blowing up images beyond their standard size and the text starts to get blurry.