MMCS Files continually downloading

I have many Apple devices and just the one PC. Twice this week I have had to delete over 1TB(!) of MMCS files that seem to be downloading continuously onto my PC via iCloud photos - I have less than 50GB of Photos there!

Not only is this taking up an enormous amount of bandwidth and presumably getting my IP twitchy about "fair usage policies" etc. This has filled my main HDD up both times, bringing my Home Office to a standstill!

I have been looking at the threads on this and Apple are notable by their silence on this. What am I missing?