I find the answer oddly comforting..............

     That evolution should select for larger brains may seem to us like, well, a no-brainer. ...
     The fact is that a jumbo brain is a jumbo drain on the body.  It's not easy to carry around, especially when encased inside a massive skull.  It's even harder to fuel.  In Homo Sapiens, the brain accounts for about 2-3 per cent of total body weight, but it consumer 25 per cent of the body's energy when the body is at rest.  By comparison, the brains of other apes require only 8 per cent of rest-time energy.  Archaic humans paid for their large brains in two ways.  Firstly, they spent more time in search of food.  Secondly, their muscles atrophied.  Like a government diverting money from defence to education, humans diverted energy from biceps to neurons.  It's hardly a foregone conclusion that this is a good strategy for survival on the savannah.  A chimpanzee can't win an argument with a Homo Sapiens, but the ape can rip the man apart like a rag doll.
     Today our big brains pay off nicely, because we can produce cars and guns that enable us to move much faster than chimps, and shoot them from a distance instead of wrestling.  But cars and guns are a recent phenomenon.  For more than 2 million years, human neural networks kept growing and growing, but apart from some flint knives and pointed sticks, humans had precious little to show for it.  What then drove forward the evolution of the massive human brain during those 2  million years?  Frankly, we don't know.

-Yuval Noah Harari,   Sapiens:  A Brief History of Humankind