Why might free space on C drive shrink by ~300MB per day

For the past several weeks I have seen my free space on C: shrink by 300 to 400 MB per day. A couple of weeks ago this had my C drive down to < 4GB, so I deleted the old version of Windows at about 35GB; resetting free space to just under 39 GB. Now free space is down to 31GB and still falling.

I have a dual drive system with my user accounts set up on D. There should be nothing on C except installed programs and system files, which I have to a large extent verified.

I have just run Disk CleanUp, and there is no significant space being taken by temporary files or other junk.

The hidden system files PageFile.sys, SwapFile.sys and HiberFil.sys are not increasing in size.

I have not been installing any new software or games over the 3 months or so of interest.

My system has currently been up for just under three days, corresponding to a drop in free space of about 1GB, and this is the top of my Task Manager details sorted by I/O Write bytes:

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Any ideas on where all this space is going?