Not all women have big thighs .. Some do and some don't. Some do because they're overweight and some are just built more muscularly.
There are some differences between boys and girls, more visible in puberty and after. Storage of fat in boys is usually in belly area, girls store fat in lower areas, usually hips/thighs. As well there are many different body types. Not all women/girls have thighs bigger than men/boys, gene perpetuation (father/mother) influence body growth - ( hormones). The reason might be to protect lower body of woman ( they give birth ) and thermo protection of abdominal area by extra mass.
they also have curves that's why.