Almost all studies show that at birth, a boy's brain is bigger than a girl's brain. At birth, the average brain of boys is between 12-20% larger than that of girls. The head circumference of boys is also larger (2%) than that of girls. However, when the size of the brain is compared to body weight at this age, there is almost no difference between boys and girls. So, a girl baby and a boy baby who weigh the same will have similar brain sizes.
In adults, the average brain weight in men is about 11-12% MORE than the average brain weight in women. Men's heads are also about 2% bigger than women's. Remember though, men on average weigh more than women and that absolute brain size may not be the best mearusre of intelligence. Many behavioral differences have been reported for men and women. For example, it has been said that women are better in certain language abilities and men are better in certain spatial abilities. Many studies have tried to find differences in the right and left cerebral hemispheres to suggest that male and female brains are different. However, few of these experiments have found meaningful differences between men and women. If fact, there are many similarities between the cerebral hemispheres of men and women.