I don't see the point of this discussion about default font & font size.

What is nice with eBook is that you can change these fonts and sizes according to your taste and your eyes. The bigger the screen, the more words you can display, or the bigger fonts you can reasonably use ... and the more comfortable you'll be. And so, eBook reading is a lot more comfortable on our NIT than on iPhone.

Obviously if pixel size is too big, small fonts will not be very smooth.
But you can compare with Sony eReader : 6" and 800x600 pixels.
If we get a tablet in 7" with 800x480, pixels are bigger, yes, but i believe it's still ok, especially if you consider that on a tablet's color screen, you can use very efficient sub-pixel font smoothing technology, not available on Kindle & eReader.