He explained this once, but it's been years and years and I don't have a source link. His argument for using the large fonts hinges on the fact that you keep your desktop monitor farther from your eyes than you do printed materials like books and magazines. Accounting for these distances, a 16-pixel or 18-pixel font size on screen is similar to an 11-point or 12-point font on printed material. Somewhere on his site, I think he has a demonstration with a photograph of a book in front of his monitor that illustrates this, and it looked pretty convincing.
This argument is unsound: CSS already takes that into account. All of the units (px, pt, mm, in, &c.) are, for screens, defined in terms of a reference pixel, which is the visual angle of one pixel on a device with a device pixel density of 96dpi and a distance from the reader of an arm’s length. https://www.w3.org/TR/css-values-4/#absolute-lengths has some more explanation and diagrams that demonstrate what’s going on.
No, the reason why it’s customary to boost the font-size a little on larger screens is because otherwise things start to look silly because you’re using such a tiny part of the space available to you, and you have a larger viewport, so you can increase the size a bit without losing too much from the screen at any time. But if you take it too far, it starts to look silly for different reasons, because it’s unreasonably large (and more importantly, inconsistent with common practice).
There is just one big problem with that - I use a Laptop and not a Desktop, which is a similar distance away from my eyes as a newspaper. The result is a website where everything looks like it has the size of the headline of a newspaper...
I guess in 2006 the resolution of monitors wasn't the same as it is today meaning (if the website wasn't updated since then) it was never tested on a 14 inch near 4k display.
