Post width change on responsive theme

I have ZigZag responsive wordpress theme by Bankic. I would like individual blog posts to open as full width posts when going directly to the page url. However, I want the main page (it’s a single scroll type site) to have two columns like it does now. You can see what I am referring to at https://debruynphotography.com The issue is that the individual posts are narrower than the content, this is slowing down the page load time as well as making the content smaller than I would like.