This is the problem with measuring screen size only by the diagonal measurement. Worked fine when everything was the same aspect ratio, but even the various 7 inch Android tablets aren't holding to the same ratio.

This is a good illustration, including the rumored smaller iPad. Even if the iPad was 7 inch instead of 7.85, it'd still have more screen area due to the 4:3 form factor.

http://yfrog.com/z/h36j2np

Ultimately it comes down to the size of tap targets. Browsing on the Fire is a PITA with web buttons/links, due to the device telling web servers it runs at a desktop resolution. Phones get around this a little by telling sites they run a pretty small resolution. I wonder what the Nexus 7 does.