The capillaries join up, and create veins.
At this point, the blood has a low pressure, as it has travelled around the whole body.
Because of this low pressure, the walls of the veins don't need to be strong or thick.
The lumen, however, needs to be large to allow the blood to flow through despite the low pressure.
Veins have valves, which help keep the blood flowing in one direction.
This is because if the blood back-tracked, it could be disasterous.