iTunes 1080p video looks better, saves space using better H.264 compression

In the comments on yesterday's AppleTV story, many people doubted whether 1080p support in the third-generation AppleTV will be very useful. That question becomes even more relevant when observing the file sizes of 720p vs 1080p versions of the same movies and TV shows in the iTunes Store: despite the fact that 1920x1080 means 2.25 times as many pixels as 1280x720, file sizes often differ by a factor of 1.5 or less. So what's going on—is the 1080p content pretty much indistinguishable from the 720p version, or did Apple succeed in magically upping the compression?
http://arstechnica.com/apple/news/2012/03/new-itunes-1080p-looks-good-through-better-h264-compression.ars