This explains pretty much why abd how to worry about this, and I think it is the first time that I actually saw the fact that a 512x512 = 2x256x2x256 = 4 × 256 x 256 calculation is implied, I hope people will understand after that than designing a game with oversized assets is non sense, and that compressed image is not an image by itself, and so must be fully uncompressed.
Also it is basically a rule of thumb: the less memory you take, and the less calculations you do, the better your game will run, and the difference between good design and bad design is that a better design will do the same job with less, while a very baddly designed game will run in "theory", but will fail in practice when everything comes into play (this is also a reason why people should try to learn those things, I don t think they need to know everything about hiw it works in practice, but at least what is a better design, and for exemple inspire themselves of old games, which were far more limited).