There are three types of blood vessel;
Arteries - strong and elastic, carry blood out of heart at high pressure.
Capillaries - very small, carry blood near to every cell to allow exchange of materials.
Veins - large lumen, takes blood back to heart.
All three have different functions.
Arteries need to be strong because the pressure of the blood would burst them otherwise.
Capillaries need to be small so they can reach every cell in the body.
Veins need to have a large lumen to allow the blood to flow in the low pressure.