A really big head.

Our brain's neurons are connected up in "small world" networks, with many local connections, and relatively few long-range connections that mostly connect "hubs". What if that were not the case? What if every neuron were connected to every other neuron? How big would our head have to be then? Well, “...a rough calculation shows that if cortical neurons were arranged on the surface of a sphere and fully connected by connections 0.1 μm in diameter, the sphere would have to be 20 km in diameter.”