Might depend on what all you've installed....like do you have .NET Framework Version 4.0? I do, but my Windows folder is only like 18.2GB on my Windows 7 Home Premium, and I have updated, today.
But, there are optional updates, and critical updates. I have all the critical updates, but only a few optional udpates.
I doubt if optional updates could add up to so many extra gigabytes, though.