Allow me to read the article for you:
As smaller and better organised brains were able to perform more complex computations, metabolically expensive larger brains simply became unnecessary
Tattersall believes that the catalyst that caused this change in thinking style was the spontaneous invention of language. This led to the neural pathways of the brain being reorganised in a more metabolically efficient way, allowing humans to get more "bang for their buck".
In other words, as smaller and better organised brains were able to perform more complex computations, metabolically expensive larger brains simply became unnecessary.