I wasn't certain about the changed font size until I realized that the FTM Forums list was taking up less space on my screen. I don't know what Apple did, but the smaller sized font is remarkably clear.
I see the same thing. I hadn't realized it until you mentioned it. Maybe this is where the retina display comes into its own...
I'm not certain.
I changed Safari 9's font size from 12pt to 9pt more than once on my Retina Mac, and I don't remember it being as clear as it is in Safari 10.
Edit: Just booted into a clone running Safari 9, and 9pt is just as clear as in Safari 10.