The 1080p video now sold on the iTunes Store manages its resolution without filesizes being as large as expected, Ars Technica notes. A 1080p video contains 2.25 times as many pixels as a 720p clip, but Ars remarks that the higher-resolution files are generally no more than 1.5 times as big. One example is Hugo, which is 3.99GB in 720p but only 4.84GB in 1080p.
