.***x files are zip files, but the method of compression is different than the standard one used by Windows Explorer. Windows Explorer does absolute compression (whatever it can compress safely, it will), MS Office and OpenXML leaves necessary pieces uncompressed to be used by the application when it is read.
Edit: I should add that you can zip the files back up and use them as an xlsx again but you have to make sure you're using the same compression method as Excel or OpenXML.
