When you run CCleaner, make a note of the cleaning that will take place.
My guess is that the Missus may do a lot of surfing/deleting of files etc. Browsers like Chrome build up a large internet cache and IE temp files grow quite quickly.
Also, when you save a file to your hard drive it is written to what is called contiguous blocks, that is, block a, block b, block c, and so on. But if your hard drive is low on space then the file system can not always write your file to contiguous blocks, and so you get files strewn everywhere, and a fragmented hard drive. Your action of a defrag puts it all back again.
I reckon.