1) on average, the brain is 8-10% larger than the female brain 2) it does not greatly affect the degree of intelligence 99% of men are the same as women 3) women have a brain 8-10% smaller due to associative centers (centers responsible for extraordinary solutions and solutions to complex logical problems), or rather, they are less developed, since they are mainly used for their intended purpose, to take care of offspring. Therefore, almost everything was invented, Nobel laureates are men. So the answer to the question is fewer neurons in the associative centers.
Women not only have smaller brains than men, but also arms, legs, bones, heart, lungs and other internal organs. And by the way, IQ is independent of brain size. The mass of the brain of a modern person is 1000-2000, the heaviest brain - 2800 - was found in a person suffering from idiocy. But if we talk about the difference in the brain between the sexes in general, then they differ in that men have more connections within the hemispheres (more developed motor skills), and women between the hemispheres (more developed analytical and intuitive thinking).