I don't know of any
direct Reading List activity would affect size on another drive.
However, Calibre does create a temporary file for each ebook sent to a device. I would speculate that what you're seeing is the space used by temporary files. Calibre deletes it's temp files when you exit Calibre, if not before; but it can leave temp files if Calibre crashes or is killed.
You can use the CALIBRE_TEMP_DIR environment variable to change where Calibre puts it's temp files. See
Customizing calibre.
Reading List stores the lists in the library database, which is in the library directory. Backing up the library directory will also backup Reading List settings and lists.