Windows Explorer, at least in recent Win versions, allows you to copy a file into the same folder with the original file, using drag-and-drop. If you had a large string of files selected in Win Explorer, and "hiccuped" so that the cursor moved while the mouse button was down, you might get duplicates of all the files that were selected. The duplicate files all go to the bottom of the list, so it's easy to not notice that it's happened.
The duplicates will all be labelled with new filenames "Copy of filename.xxx," It's even possible to do it multiple times and get "Copy 2 of filename" duplicates etc.
In recent versions you can do a Win Explorer search for Filenames Containing "Copy" (without the quotes) and the search result will show all such "duplications." If you're sure that's what they are, you can Ctl-A to "Select All" in the search result window and hit Delete. Older Win Explorer versions may or may not accept wildcards in the "Search For" or "Find" specification, but you should at least be able to tell if that's what produced your large jump in drive content.
It works with folders too, so a folder with a lot of stuff in it could have been duplicated as "Copy n of foldername."
It's the most obvious thing that might cause a huge jump in drive content, although it depends a bit on which Win version you're using.