Solved Disk space fluctuations and average size of Windows root folder

When I'm nearing the threshold of adequate free disk space for reasonable defragmentation times, I start to monitor my disk space utilization a bit more closely.

Sometimes I notice some strange changes in how much free disk space I have, meaning fluctuations in amounts as high has 3 to 4 gigabytes. For example, I will see I've got 21.6Gb free then suddenly notice it has dropped to 17.9Gb. I use the Disk Clean utility to flush any excess files out, but that doesn't do much. I check the temp folders manually, just to be sure there aren't large garbage files left behind either. I clear the cache of my two main browers. Still no change. I'll go run Windows Update, expecting that maybe some files were downloaded and are now waiting to be installed, but that doesn't turn up much either. I even reboot to see if that forces some kind of clean-up (which sometimes happens), but that may not free up any disk space.

Later on, I'll check again, and sure enough I'm back up above 20Gb in free space. A mystery, as I've no clue as to what changed. Does anyone have an idea as to what may account for such large amounts of disk space being mysteriously occupied and then released?

Also, I just checked my Windows root folder and see it is taking up 11Gb of space. I seem to recall some time back when I first got my system that it was not much above 8Gb. Why would the operating system file tree end up swelling by nearly 50% and is there anything within it that could be pruned without harming the system? I've read up on shadowstorage and that sometimes Windows will fill up more restore points than it really needs. But I have just two restore points, nothing more.

Hi Again
I think that the best way is to test yourself. Download win dir stat and it will give you a complete diagrahm of what is on your hard drive at any time.
WinDirStat - Windows Directory Statistics
and
make the mandatory malwarebytes and anti virus scan.

In addition, to the above it may be
This could be due to c:\pagefile.sys or c:\hiberfil.sys that can take up
several GB depending on your size of RAM and other parameters.
Crash Dumps can take various amount of space too. Crash dumps can be remove with ccleaner. It does a great job at removing crap.
or more likely
System Restore points

Yes, I have run malwarebytes periodically and always have anti-virus running with frequent updates (I use Avast!, which seems to be more robust than AVG Free).

I also have a fixed allocated swap file size of 4Gb, so there's no shrinkage/swelling in that area.

I just ran another Windows update, which downloaded about 10Mb of files. When the process completed... guess what? I'm back up to 21.6Gb free! Very weird. Anyway, I haven't tried using WinDirStat before (although I'd seen it mentioned here and there). Will give it a try.

I can't really figure it out... but, it's a non-critical thing really, as long as ample disk space is available.