I think lots of people are comfortable with smaller fonts, but I find myself genuinely straining my eyes too much and getting headaches if I go smaller than this, and I already wear glasses (although I should probably update my prescription).
Oh, there's no "right answer" to font size, but the fact that my size would work on a 1200p screen (and many of my coworkers have significantly larger screens and younger eyes than I) could go towards explaining why the sidebar is on by default and the gutter is so huge.
