Iljitsch van Beijnum for Ars Technica:

…despite the fact that 1920×1080 means 2.25 times as many pixels as 1280×720, 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?

Van Beijnum concludes that Apple’s compression for 1080p content has improved. That, combined with specialized decoding hardware in the new Apple TV and the new iPad, makes it possible for that video to look great without making the file itself huge.