I'm going to have to second this. Why waste all of this screen real estate? I zoomed it out to 67% on Chrome to make it look normal. Definitely one of my biggest pet peeves about modern web UI.
I think the idea behind the bigger font sizes is that the space between your head and the screen is typically greater than that between and a book you're reading. I would agree that this takes a bit far, but it may look nice on a mobile device.