It would not surprise me to see little difference in the stored file sizes.
MS certainly realized that a significant file size increase in all the Office
apps would be an issue with many customers. So the XML is actually
compressed within the file. But I still have to believe all that code that
takes what used to be interpretted for display from the XLS format, now takes
a lot of new code to interpret an entirely new (additional) format.
Here is a good article on the file format, albeit rather basic.
http://office-watch.com/office/archtemplate.asp?v10-n18
Section 1,3,4,5 cover it pretty well.