Look at modern game files in your computer, they are locked and compressed archive files, you can't rummage through them at your leisure. Patch Installer can't simply go inside the file and replace some code or model. It has to replace entire file and they tend to be in GB size range these days.I've honestly never understood why patches are so large, maybe someone can fill in the gaps for me.
You see, most patches are simply bug fixes.. stabilization, optimization, etc. That's all program code, and the core program that is actually "the game" is actually very small. TW2's actual executable is only around 15MB. The vast majority of what's on the disc is assets.. meshes, textures, etc.
So what's in a patch that's 6-7GB? Some games have patches that are 20GB+. There has to be an unusually large number of raw assets in a patch that size, why on earth are they still working on assets at the eleventh hour? And later on, devs release patches that are literally just bug fixes, and yet it still ends up 5-10GB in size. What the hell?