If .xlsx is a zip archive, why does zipping it reduce file size?

[Welsh Wizard]

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 aren’t Microsoft using a more efficient compression technique?
File size can be a problem when trying to email, for example, multiple
versions of the file.

 

[Bernard Liengme]

This is taken from my notebook - sorry I did not record its source
----------------------------

To reduce the workbook size:

1. Press Ctrl+End to find the last cell in the used area within the sheet.
In the screenshot, the last cell is E17.
2. Find the last cell containing data in the sheet. In the screenshot, the
last cell containing data is cell C11.
3. Delete all the rows between the cells containing data to the row of the
last cell in the used area. In the screenshot, the rows to delete are 12:17.
4. Delete all columns to the right of the column of the last cell containing
data, up to the column of the last cell in the area used. In the screenshot,
the columns to delete are D:E.
5. To quickly delete the rows, select the first row to delete (row 12 in the
screenshot), press Ctrl+Shift+Down Arrow. To quickly delete the columns,
select the first column to delete (column D in the screenshot), and press
Ctrl+Shift+Right Arrow, press Shift+F10 and from the shortcut menu, press
Delete.
6. Repeat the steps above for each sheet in the entire workbook.
7. Press Ctrl+S to save the file.
-------------------------------
best wishes
Bernard V Liengme
Microsoft Excel MVP
http://people.stfx.ca/bliengme
remove caps from email

 

[Welsh Wizard]

You haven't ansered the question at all - see subject line and my final
paragraph. I thought I made it clear that I had taken all steps, such as you
suggest, to reduce file size. I am not looking for file size reduction advice
in terms of the Excel contents, though others mght not know how to do this.

 

[Bernard Liengme]

I apologize! Just want to be sure you had minimized file size,
best wishes and peace

 

[Dave Peterson]

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.

 

[Dave Peterson]

Oops.

The programs ask you if you want the smallest size and slowest speed or larger
size and faster speed.