Well if you have a 512x512 icon uncompressed it is an even megabyte, so that makes the calculations fairly easy.

But raw imagery is one of the few cases where you can legitimately require large amounts of RAM because of the squaring nature of area. You only need that raw state in a limited number of situations where you are manipulating the data though. If you are dealing with images without descending to pixels then there's pretty much no reason to keep it all floating around in that form, You generally don't have more than a hundred icons onscreen, and once you start fetching data from the slowest RAM in your machine you get pretty decent speed gains from using decompression than trying to move the uncompressed form around.