Purely biologically speaking it is a known trade-off. Intelligence allows tool usage and abstract reasoning about one's surroundings, but it comes of the cost of using up fantastic amounts of energy, which is why even in humans intelligence is not continuously used at the highest level of processing.
Don't forget higher child infant mortality and premature births. Humans are born several months earlier than optimal in order to be able to fit that huge head of ours through the birth canal.
That's not my understanding of the modern interpretation of head size and pelvis size. I believe current thinking is that the (healthy) mother simply cannot metabolise nutrients efficiently enough to sustain further growth of the (healthy) foetus beyond a certain size. Evolutionarily, the pelvis has adapted as far as necessary, and no further.