No and no.
That's NOT how Steam patching works, and retail patches often are even slightly larger due to additional stuff like pictures or even music for the installer, additional stuff that nobody needs (such as xfire included in many patches) etc.
Steam usually gets told to download and replace x files. BIS simply hits Ctrl+A and finished their Steam patch.

Also, people should defrag their drives after each ArmA update on Steam, as they spread out your ArmA2 files over your entire hdd, increasing load-times by quite a lot.