What game plays off the disc itself? Most games copy to the ssd and then just download a new full copy of the game from the cloud the second the copy from the disc is done.
That is if there isn't a patch, otherwise it will just download most of it again like you said.
Why do games need to download textures, sounds, videos over again? I just can't understand why a 100GB game has a 60GB patch etc, other than purposeful obfuscation or complete disregard for care.
AIUI the process of building asset packs and compressing them can mean that modifying a single texture can mean complete reshuffling of the resulting file; and that means redownloading the whole compiled "pack".