It's common that human females have a more flexible body than males, (but I don't know why, so it'd be helpful if someone answers why so as well!) but does this apply to all mammals?
Try birthing a child for once. You need to be flexible to accommodate the growth of another organism inside you.
Take a look at some pictures of the mothers spine during the course of a pregnancy, you will have no further questions. Same with belly size, breast size and vagina size upon birth.
This video is a nice explanation of this. Also includes the different centers of gravity for males and females (human).