RoughlyDrafted notes that the native size of the new bonus content is 1280x720. Clearly this isn't aimed at mobile devices. In fact the content even needs to be scaled down to fit in an iTunes window on the 1280x800 13" MacBook.
Apple likely chose 1280x720 so that the content would play natively at an HDTV resolution. The exact resolution of the Apple TV.
The Apple TV was called sort of a new DVD player for the Internet age by Steve Jobs and iTunes Extra closes the gap in features between the two. Users will now be able to sit down in front of the TV and browse interactive bonus content just like they would with a DVD rental or purchase.
If you look at the headers found in the index.html of a iTunes Extras bundle you will see these clear references to a HDTV.
Hopefully an Apple TV firmware update will come in the near future to enable the use of these new features.