Anyway, at the apparent risk of being on the receiving end of some down votes myself, I'm more curious why giant fonts are a trend. I'm old, I wear progressive lens (what used to be known as "bifocals"), and I still hit the Cmd⌘-+- combo a few times to crank it down to a readable size. Is it some attempt to capture the aging baby boomer market? (I ask with tongue somewhat in cheek...)
Also, until recently most browsers didn't support subpixel typographic adjustments (for instance, "letter-spacing: 0.4px"), and you could only adjust this stuff reliably if your font-size was higher than 20px.
That said, 16px is the default base font size on most user agents, and it seems adequate for reading on many devices. However, I tested 16px in Luis's site and it was not quite satisfactory. 19px seems to be the sweet spot in this case.
Ultimately, no font-size is perfect for everyone. Be thankful that your browser zoom works ;) Even on a desktop, sometimes at night, with f.lux on, I have to zoom HN to read it comfortably (thankfully Safari has tap to zoom in an area).
If you imagine that page on a phone screen, you'd find that the font size is probably a lot more normal than it is large. If you're expecting > 50% of traffic on mobile then that's the use-case you optimise for.