Hi Jo8192. Customizr is responsive, which means that as you change your browser width, it will adapt. If you reduce your browser window size, you can see it adapting to the window. At some points, it adapts in “jumps” (because it has various breakpoints at 1200px, 979px, 767px, 480px, and 320px). Your browser window looks to be around one of those “jumps”, where the margins are at their maximum. If you increase or decrease the width slightly, you should see them reduce.
This is working as designed: most people will see a pleasing balance, but every now and then someone will have a browser window width that looks different. The person I wrote my site for only has a laptop and the slider takes up the whole window on his PC; others complain that the pictures aren’t big enough in the lightbox because they have massive screens—it’s impossible to be perfect in every sized window.