First, I have no idea.

Second, I know that there are zip programs that ask you want you want--smallest
file size and fastest speed or larger size and slower speed (and options between
these two).

Maybe it's just a choice that MS made to keep the time to create/update the file
reasonable.



Welsh Wizard wrote:

I have a large and complex Excel workbook, with perhaps 20 large sheets. All
redundant data, cells etc have been removed.
When saves as .xlsx it occupies almost 3MB of disk space. However, if I
compress it to a .zip or a .rar, using any standard tool, it reduces to about
25% of that, under 700MB.
As .xlsx is already a zip archive, why can it be so massively further
reduced in size? Zipping a zip usually has no effect or even increases the
size. Why arent Microsoft using a more efficient compression technique?
File size can be a problem when trying to email, for example, multiple
versions of the file.

--

Dave Peterson