i am not sure about android, how it works. but i think, in android also, except a few apps, all the apps have to downloaded completely. i remember updating Candy crush and subway surfers on my wife's S3. had to download full app.
but generally, in all mobile / phone OSs, the apps are in complete bundle formats, not in parts. (i am considering WP, symbian, iOS, WinMobile, etc.)
thus, when u update, u have to do it completely.