It was Big Sur that focussed attention on the size of macOS updates. In Mojave and earlier, updates had essentially been Installer packages that brought a minimum of overhead. By the time that many had installed Big Sur’s new Signed System Volume (SSV), we were starting to discover just how large its updates were. Those were early days with its completely new updating process that builds a new System volume, takes a snapshot of it, and constructs a tree of hashes that verify the integrity of every last bit within it.
↫ Howard Oakley
I had no idea macOS updates had become as big as they had, and good on Apple for trying to bring this back down again, and speed up installation of updates, too.
