Arteries are thicker than veins give reason.

1. Arteries have relatively thick walls to enable them to withstand the high pressure of blood ejected from the heart. 
2. Arteries expand when the pressure increases as the heart pushes blood our but then recoil when the pressure decreases when the heart relaxes between heartbeats. 
3. This expansion and recoiling occurs to maintain a smooth blood flow. 
4. Veins, on the other hand, have thinner walls and larger lumen-veins have no need for thick walls as they need not have to withstand high pressure like arteries. 
5. Moreover, as veins transport relatively low Pressure blood, they are commonly equipped with valves to promote the unidirectional flow of blood towards the heart.