You widen the single-post, maybe add inner padding, increase line-height, font-size. It just looks weird otherwise, the post is too narrow. Make it look like its comfortable in its environment instead of very shy and afraid to take any space.

I would prefer the margins narrow to no more than 10% of the page width. The left margin already has some navigation in it, fine, the right margin doesn't need to exist. If that means blank space below the article I'm ok with that. Having to scroll down to read an artificially skinny single post is, in my opinion, bad UX.

I find it difficult to read articles that are wider. There are a number of readability studies that back this up, it seems that 50-75 characters (including spaces) is a sweet spot for readability, and that things get terrible pretty quickly once you start increasing the line length. This design is 80 characters wide or so, putting it at the upper limit of reasonable width.

I would rather have to scroll down on an article that I can actually read, rather than fit the article on my screen but be unable to read it well.

Wide text only really works for very short text of a few lines. Long text comments on HN tend to suck pretty badly with wide browser windows.

Maybe ads? But I got adblock, so I don't know

You would rather see ads than whitespace?

Maybe recommended posts instead?