Keynote file doesn't reduce in size after deleting slides
I created a Keynote template with several Master slides containing images, graphics and text and the final size was 17MB.
I removed a couple of the masters and realised the size wasn't reducing at all.
Eventually I deleted all the slides and masters, leaving 1 empty white page and resaved the file but it remains at 17MB.
I've tried copying slides to a new file, reducing individual image sizes, but can't find an intuitive solution.
Any help with this would be much appreciated.
MacBook Pro (Retina, 15-inch, Late 2013), OS X Yosemite (10.10.4)