updated 02:10 pm EST, Fri March 9, 2012
Newer hardware required for maximum resolution
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.
The answer is said to involve H.264 compression. Apple's official specifications show the new Apple TV as supporting "High" or "Main" profiles up to level 4.0, while the new iPad and the iPhone 4S can operate up to level 4.1. It's thought that the High profile is allowing for more efficient decompression than that used in Main or "Baseline" profiles, common among older Apple devices. The iPad 2, for instance can only function up to Main Profile level 3.1. On Macs and PCs, iTunes requires a v10.6 update to support 1080p.