Windows updates are kind of snapshots, they replace more files than necessary and keep the old files incase the update fails or you wish to roll back x update. Besides that you're also given new packages and features you never asked for, because Windows loves their guinea pigs and doesn't care if something breaks because of it.

AFAIK the main reason is in how windows handles the filesystem - in linux everything is a file and all files are cached by default unless that memory is needed by default, so 100% memory utilization is the norm and where Linux operates most efficiently. In windows file caching seems architecturally be an after-thought and much less efficient - i.e. this causes handling a lot of files (like when updating the OS, where a lot of files need to be modified) to break the caching system and cause a lot of cache thrashing.

While I'm unsure of the actual reasons, the file system alone seems too be a pain.