Your arteries pump blood away from the heart at a relatively high pressure. Veins on the other hand return blood to the heart at a lower pressure and must do their job against the force of gravity. That’s why it can feel so good to put your feet up at the end of the day. To help the correct flow of blood back to the heart the veins have valves, flaps of tissue in the vessel that allow the blood to move only one direction. Over time and with certain other factors these valves can become damaged.
This lower pressure, gravity, history of long periods of standing (typically seen in certain occupations), damaged valves, along with genetic pre-disposition all contribute to the improper reverse flow of your blood. This is known as venous insufficiency. The reverse flow, or reflux, directly causes more blood to be in the vessel. This increased amount of blood further increases pressure and makes the vein larger over time.