Thanks for your thoughts, Spurlash2. That's one of the problems, Madame deletes absolutely nothing! She does seem to choose Chrome (another issue!) but doesn't surf a lot at all. She mainly plays hour upon hour of online bridge. I only recently stopped deleting her internet cache and some other things because she never closes anything - often having well over 30 tabs open at once - and gets upset whenever my cleaning means that she opens Chrome to find an empty screen....
In any event I don't think that the accumulated internet cache could ever approach 30Gb in a week. The other consideration is that her computing behaviour absolutely doesn't change from one week to the next so these enormous swings couldn't be explained in that way.
I'm not convinced by your other point either (sorry!), assuming I have understood it correctly, because the fragmentation to which you rightly refer doesn't really consume extra space but just slows everything down because the head has to chase around so much more to assemble what is required. You can confirm that by watching the free/used space figures or the pie chart as the defragmenter does its work. Moving blocks about to new and more efficient locations doesn't significantly alter the total space used for the data. I'd wondered if the first time the drop in used space occurred because I had chosen the option to wipe the free space, almost as if the unwiped free space had in some way been recognised as full, but this last time the same huge fall happened without wiping.
There must be an explanation, though.