Over-The-Air iOS Software Update Saves 700MB of Downloading
iOS 5 Beta 4 is the first software update to be downloadable using Apple's new over-the-air updating system. As noted by @iam_lost, "iOS 5 beta 4 over-the-air update is a mere 133 MB, compared to the chunky 824 MB .ipsw file."
The over-the-air software update only downloads code that has changed or been updated in the update, thus making for a much smaller download. In this case, 691 MB smaller.
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http://www.macrumors.com/2011/07/22/over-the-air-software-update-saves-700mb-of-downloading/