Why is the diameter of an artery typically smaller than that of a vein?
Arteries contein high pressure blood and veins don't.
Arteries are the vessels that provide blood from the heart to the rest of the body. Because of this, the blood needs to flow at a higher presure to make sure that it reaches every cell as fast as possible. This is why arteries have a small diameter compared to veins, also arteries are surrounded by a thick muscular layer that allows them to modify their diameter.
On the other hand, when blood reaches the veins, it is coming from capillaries which have the smallest diameter, so blood presure decreases while flowing in them, resulting on a wider and more irregular cavity.