I do not believe women (for the most part) are aggressive or violent in nature. They are nurturers and caregivers.
You have obviously never met a mama defending her children then! Human females were bred to be more submissive, however, in nature it is very common for women to be aggressive. Lionesses do the majority of the hunting. A mama bear will stop at nothing to defend her cubs. And don't you dare touch a hamster's babies and expect not to be bit. There are quite a few species that have larger and more aggressive females than males, especially in insects. The males do the occasional fight for females. Can you imagine what a woman on the front lines would do if they thought they were defending their family? Isn't that the reason a lot of men go to war and become aggressive as well, to defend their family (country)?
We as humans have just drifted towards smaller and weaker women, and therefore society will view them as such even when they're not. Perhaps women are biologically weaker as well, it is possible. However, it is because that is what society has desired. In just the same way, society could start desiring stronger and more masculine females and in result, we would end up with more buff women.
Something to think about: Biologically, every person starts out as a female. It's just a few chemicals that choose how the body is shaped. Testosterone does seem to cause some aggression though, but then again any woman knows those female chemicals can do the same!