Reduce Keynote file size

Whenever a custom theme is applied to a Keynote presentation, several unused theme files will be added automatically (Unzip the keynote file and look in the theme-files directory). This makes the file size 5-6 MB larger. 

So, to have a small keynote file, do the following
1. Select a plain Keynote theme, e.g., White
2. Customize the look and feel of slides as desired.
3. Save the file as a normal Keynote presentation, say template.key
The file size should be about a few hundred KB.
Disable "Include Preview in the document". Otherwise the file will be large.
4. Create a new presentation from template.key, not from a theme chooser.
This prevents Keynote from saving unused theme files in the presentation and 
will keep the presentation file size small. 

A keynote file is actually just a compressed folder.
For an existing Keynote file with large size, you can unzip the keynote file and look inside 
to see if 
- it contains a large preview file
- it contains theme files  
- it includes large image files 

Unused theme files can be deleted and touched to make it zero bytes with the following bash script:

#!/bin/bash

for file in $(ls stone*.jpg); do

  echo "Processing $file ... "

  rm -f $file

  touch $file

done

Then, compressed the keynote folder and rename the zip file back to .key extension.