Question

the human size is bigger in vein when compared with artery why​

Answer 1

Answer:

Veins carry more blood volume so they are generally larger in diameter whereas arteries are smaller

Answer 2

Here is your answer

The veins which carry deoxygenated blood to the heart have wider lumen so as to carry large volume of blood from all around the body to heart. The blood moves inside in a very low pressure so the wall of vein is thin and it contains valve to prevent the back flow of blood.

Whereas the arteries carry oxygenated blood from the heart to the body under high pressure due to the ventricular contraction of heart. So the walls are thick to sustain the pressure and have no valves so that the blood reach to the farthest body part without any obstacles.

hope it's helpful