Jesus Diaz put up a fascinating comparison of the 512×512 icons Apple currently uses on the Mac and iOS devices and the 16×16 icons Apple used on the original Mac.
Here are some more staggering numbers: 28 years ago, a monochrome icon on the Mac was 16 x 16 pixels. It only took 32 bytes of memory. Compare to the 512 x 512 pixels of each iOS’ icon (required to go on the App Store). It takes four times the total video memory of the original Mac to represent a single icon in iOS at full size. Of course, iOS’ icons are not shown on screen at that size. They are much smaller—114 x 114 on the iPad. Eventually, however, you can be sure that there will be displays that would require that insane pixel density.