I feel it is not just about bigger but also about what all is supported in the current brain that is potentially not as useful anymore or useful for intelligence as such. The evolutionary path for our brain has an absolutely major focus on keeping itself alive and based on that keeping the organism alive. Humans will often take potentially sub-optimal decisions because the optimal decision may have a very low probability of death for themselves or those genetically related to them. In some sense, similar to a manned fighter jet vs a drone, where in one case a large amount of effort and detail is expended on keeping the operating envelope consistent with keeping the human alive, whereas a drone can expand the envelope way more because the human is no longer a concern. If we could jettison some of the evolutionary baggage of the brain, it could potentially do so much more even within the same space.