High Profile Vs Main
The reason that the 1080p versions of the iTunes Store videos can be a good deal better without doubling the file size--or worse--can be found in the tech specs of the new AppleTV and the new iPad. The AppleTV now supports H.264 compression for 1920x1080 resolution video at 30 frames per second using High or Main Profile up to level 4.0, the iPad and the iPhone 4S the same up to level 4.1. The profile indicates what kind of decompression algorithms the H.264 decoder has on board--the "High" profile obviously has some tricks up its sleeve that the "Main" or "Baseline" profiles known to previous devices don't support. The level value indicates how many blocks or bits per second a device can handle.
This is a bunch of misinformed garbage.
This has everything to do with how the video was encoded.
The video was re-encoded with "high profile". The decoder must support high profile to decode the video. High profile gives better quality at the same resolution and allows you to increase resolution of the output without increasing file size dramatically.
The original content was probably 1080p or 1080i and was scaled down to 720p. Using high profile and no down-scaling allows better quality with only a marginal increase in file size.
The amount of computation required to decode high profile vs main profile is significant if done in software. They are probably using the additional two GPUs on the A5x to do the decode. iOS devices don't have dedicated chips for video decode. It's done either by the GPU or the CPU. Which is why, up until the release of the iPad(Generation 3), iOS devices have not been able to support high profile streams. They run out of CPU/GPU cycles.
Yea, they added software for decode on the devices but it's not just enabling it on the device and there are no tricks in the decoder. The tricks are done in the encoder to get the file size down.