As we know Apple released iOS 5 beta 4 not that long ago adding one of the most iOS 5 popular and wanted feature: Over-the-air-updates; And the fourth beta version of iOS 5 was the first firmware/software update to use this feature. If you have tried updating your  device to beta 4, you would noticed how fast it is to update compare to updating your device through iTunes. As noted by twitter user @iam_lost, the iOS 5 beta 4 is just 133MB, compared to the IPSW downloaded through iTunes which is 824MB.

iOS 5 Beta 4 OTA

The reason the OTA (over-the-air) update is much less smaller than the regular update through iTunes is because the OTA software update only download code that has been changed or updated.

(Via MacRumors)