First thank you for your response and sorry for my confusing question. Well, I'm not good in English, so it's a little hard for me to explained it more detail. But, here are some examples/explanations which maybe can help you to understand more of my question.
Some references says that our body proportion have a basis, and it must be fullfilled to be called an ideal body. Like Leonardo Da Vinci in his Vitruvian Man painted in rectangtular and circle line, or 7-8 heads theorem which counts the body size using heads as the unit, etc.
Another example is like what @saab said above....
What I want to know is simply why ??
You said that if our leg bones get so long it will be fragile, right? then in your case, I want to know, what is the ideal length and why??? in what basis you can say that it's too long or it's to short.... Same case with the hand, body, and etc lengths.
Another example, in my first post I said that why our leg have to be longer than our hands... What's wrong if the hands is longer??? who knows maybe in the past it is supposed to be like that.
When you said "stable body proportions", means there must be some researchs that had been done.... That's why they can know It's the most stable, right??
I want to know the reasons..... that's all.