One big reason is, women have lower testosterone levels than men and certain other genetic differences that make it much more difficult for them to put big muscle mass on. That being said women can progressively lose body fat and increase lean muscle mass just as men.
Women achieve big bulky muscles only by choice. They achieve this only through extraordinary means, like steroids.
Women can workout 6 times a week and lift some serious iron, and the results will be a sexy, toned and fit body, thats how women are genetically built.
So go pump some iron ladies!!!