But what I'm trying to say is that I think you're looking at it from the wrong perspective. It's not the margins that are too wide, it's the text that's too narrow.
This may sound like the same thing but it's not. It doesn't matter what percent of the page is taken up by margins, so long as the physical line length vs. font size is comfortable to read. Too long and it's harder to follow, too short and, as you say, you have to scroll constantly. If you had the same physical text width but a smaller font, it'd be better (so long as the font was still big enough to read comfortably).
Classical research suggests the ideal line length is somewhere between 45 to 75 characters. For the web it's suggested you can go up to 85 characters, depending on the font. This website sets the maximum width at 36.5em which is on the narrow side of things.