[femto113]

I believe this is because Apple wants the logical size to correspond (roughly) with physical size. This is particularly important for user interfaces. A button needs to be sized for the human finger that is tapping it and thus should be the same physical size on each device, which in turn means it should take up a smaller percentage of the logical size of the bigger devices. The only case I know of where they broke this was the iPad mini, which has the same logical size as the larger iPad, and you may notice that everything on the iPad mini is just smaller.

[sirn]

To be fair, iPad Mini has the same PPI as the original iPhone (163 PPI) and iPad Mini Retina has the same PPI as the Retina iPhone (326 PPI) so it could be argued that iPad is the only device where everything appears larger than the rest.