You seemed to be suggesting that it had something to do with margins, which is actually a different issue than column width. In a newspaper, you can have a lot of columns with small margins, for example.
You also seemed to be suggesting that it had something to do with separating different stories on the same page.
When you have one column of text, as in the frontpage of HN, large margins implies a narrow column of text.
My point about separate stories is that it is independent of them. Magazines and newspapers layout their stories in columns. I was anticipating someone saying "Yes, but they also show multiple stories on one page." But magazines and newspapers still use narrow columns of text even when they have a single story on the page.
You also seemed to be suggesting that it had something to do with separating different stories on the same page.