My guess is just that evolution is dumb. Larger brains are less efficient (compared to small brains like birds have), but the cost of a less efficient brain is not so heavy in larger creatures, so there has been less selective pressure for brain efficiency in larger creatures.
My guess is just that evolution is dumb. Larger brains are less efficient (compared to small brains like birds have), but the cost of a less efficient brain is not so heavy in larger creatures, so there has been less selective pressure for brain efficiency in larger creatures.