Can't reduce Keynote file size

I had a 17MB template that I wanted to reduce. I started removing slides to identify which was causing problems, but even with just one remaining blank slide, the file remains at 17MB.
I've restarted Keynote and my Mac, but the file won't reduce in size.
Any help would be much appreciated!

The template probably contains some art assets (like the other slide templates, for example). I created a test document in the latest version of Keynote choosing a plain white slide template (the second default template) and did nothing to it but "Save As" (which, as you probably know, you access by holding the option key while opening the File menu). The resulting saved document was 373KB.

Thanks Chas.

I've deleted all my graphic/image heavy master slides too and for some reason the one remaining slide - with 2 words on it (!) - is 17MB...