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The problem comes from the fact that the container is set to a fixed 1100px width instead of a max-width, so it can scale down.
.page-id-2145 #container {width: 1100px;margin-left:auto;margin-right:auto;}
What I am not sure is if you did that selecting an option or entering a CSS rule.
Check how you setup the Container width and if you find it correct that.
If you cant find it, we could make an overriding CSS rule, but it would be better to fix the source