Don't forget that publishers can set a default font size in their eBooks. Though it is better not to do that IMO, if they do then the font sizes can vary from eBooks where no size was set. The only way to know for sure if the new Kindles render font sizes different from the older models is to load the exact same book on each model and then compare them. Trying to judge this from a photo of a displayed book when you know nothing about that book's formatting is pointless. For all we know it could be a TOPAZ book or a sideloaded mobi that was modified in calibre.