I'm not sure the point of this question - a brain size larger than a human's doesn't matter that much if the extra size isn't going to the right places. From what I can tell it's the size of the brain dedicated to higher cognition (cerebral cortex) and the "power" of the neurons it contains (humans spend more relatively more energy on their brain than an animal's brain of the same size). The answer to your question (how to make AGI dumber than humans) simply seems to be have fewer neurons in total, fewer neurons doing higher cognition, and have those neurons be less powerful.