I think part of the reason we are seeing larger font sizes across the web is that web designers and users are placing more importance on the clarity of content. Sites like Medium or Smashing Magazine, for example, often publish articles with large swathes of content relating to their subject, and larger text is par for the course - it keeps things clean, but legible and accessible, and we have way more web font choices than we ever used to have.
The old '10-12px Verdana' paragraph font sizes of Web 1.0. just don't cut it any more, on almost every level: accessibility, legibility, clarity, adaptability (in a responsive web context) and, last but not least, aesthetic beauty. I love that web designers are treating web typography with a greater respect now than they ever have; it is extremely important to our medium, and to cut a long story short, we have the technology. ;)