My second animation tip: KML embedded COLLADA objects can be animated by using the time primitives similar to what I did for prism maps.
The advantage is smaller file size compared to prism maps, since only one COLLADA model is used. Still you have to link in this model for all countries in all years (around 4000 times in this animation). A better way would be to be change the size parameters dynamically, but this is so far not possible with KML.
Download KMZ archives here and here:
Population statistics from UNdata.