Why does a 170 Kb file becomes a 3.5 Mb when I save it as theme?

Question

I realized that when I save a file (170 Kb) as theme, its size increases dramatically (3.5Mb) (tried in keynote 09 ad 08 under OSX 10.6.7 or 10.5.8)
I realized that by doing "Save as" > Advanced Options > select "Save theme image into document", the saved file becomes a 3.5 Mb file as well.  
Therefore, I suspect that the "save theme" does by default select this "Save theme image into document" option, hence the 3.5 Mb file size.
I tried this on several Macs and got the same result. However, all Macs are Macs I used at some points or where people installed my files / themes.
As a result, all files created from the theme I created are big fat babies of 3.5 Mb, which defeat the purpose of a theme to make it clean and lean.
Please advise if you have the same issue.
Saved custom themes can be found at [Home]/Library/Application Support/iWork/Keynote/Themes.
Suggestions welcome on what caused this oversized theme file size and how to reduce it?
Thanks in advance

Answer 1

OK, problem solved. I was having this same issue & every presentation that I made from the new template was over 3.5MB, even if it contained only text. I now have it down to 229kB.
I used a plain black theme as my basis for a custom theme. I modified the fonts, layouts, etc, just as I wanted them. Then I saved as a template. The file size balloned to 3.5MB. The reason turns out that the default charts fot the black theme have woodgrain surface textures & these are being saved into the custom theme file (in a folder called "theme-files" within the custom theme package). So the solution is to remove all references to textured charts in the custom theme.
So... Get your custom theme ready & save it as a regular presentation somewhere for safe keeping. Now create a new chart on a new slide. (NB. A new regular slide, not a Master Slide.) For simplicity, choose the top-left-most chart type (column chart). Open the Inspector; go to the chart tab; select Chart Colors. In the new Chart Colors window, select 2D Color Fills & pick any colour. Click Apply All. Go to the menubar item Format --> Advanced --> Define Column Chart for All Masters. Click OK with the default option of "6".
Now, use the Inspector to change the chart to the next chart type along. Repeat the process above of changing the chart colours to a 2D colour fill, then applying this to all masters. Repeat, repeat, repeat for all chart types. I couldn't select a colour fill for some chart types, such as the line graphs.
Once you've defined all charts as using a 2D colour fill, there are no references to textures any longer. So, now delete the chart & its slide. Go to File --> Save Theme. Give it a name.
Now, check your custom theme file size in the Finder. Hopefully, it's come down in size now!