I believe that’s because your theme was built responsively. So it will adapt to its browser size. Thus, you’re getting such layout.
If you don’t like it, you can try removing/altering the code by removing this line of code,
<meta name="viewport" content="width=device-width, initial-scale=1.0">
I don’t know if its gonna work, but I suppose that it should. You can try it to be sure.