what is the difference in trend of Body mass Index between males and females?

Answer 1

Because of difference in genetics and hormones, and the resulting metabolic and physiological differences (especially from estrogen and testosterone levels), males have significantly lower BMI than females. Though the average range over both genders is 18.5 - 24.9 kg/m^2, men can often safely go below 16, whereas that can cause loss of fertility in women.

Answer 2

"Healthy" males will tend to have a lower BMI than "healthy" females due to the fact that males naturally have a lower percent body fat