Losing Our Ping Pong Balls

Anthropologist's at Dartmouth College have made a discovery after been digging around inside the skull's of humans who have died over the last thousands of years and found that we may be the most advanced civilisation ever exist on Earth, but we are doing it all with smaller brains.
They found that around 100 generations ago, our ancestors had larger brains and you may look around at some of the knuckle draggers walking around today and say 'no shit Sherlock' although another report from Norwegian researchers found that IQ scores have been steadily falling since the 70's.
Obviously brain size does not mean a higher IQ, on average the male brain is 10% larger than the female brain and look at the bang up job they have done but back to the Anthropologists who decided to use the medium of ping pong balls to explain the volume of grey matter we have lost since our great, great, great, great...x100 strolled around with their massive brains.
Apparently the amount is four ping pong balls worth and as i am not up to date with the exact International Table Tennis Federation's rules on ball size, i googled it to find that one ball is 40mm so the volume lost is 160mm which is quite a bit from a 1500mm brain today.
So what happened around 3,000 years ago to mean that we needed an extra 9% of our brains volume?
It could be our diets changed or the increased interaction with other humans as civilisations began to come together but basically the men with the trowels and dirty knees didn't know but they are certain that brain size does not matter and i would agree, the Sperm Whale has the biggest brain of any animal species, 800,000mm or 20,000 ping pong balls, and they haven't invented anything.