The most profound difference between girls and boys is not in any brain structure per se, but rather in the sequence of development of the various brain regions. The different regions of the brain develop in a different sequence, and different tempo, in girls compared with boys.
Girls reach the inflection point (halfway in brain development) just before age 11 years; boys do not reach the inflection point until just before age 15 years.
A young woman reaches full maturity, in terms of brain development, between 21 and 22 years of age. A young man does not reach full maturity, in terms of brain development, until nearly 30 years of age. Differences between the brains of adult women compared with adult men are small.
Source: “Sexual dimorphism of brain developmental trajectories during childhood and adolescence.” Lenroot et al.