The recurrence of varicose veins may be due to the saphenous vein stump left after dissection or laser (laser always leaves the stump for fear of burning the femoral vein). The saphenous vein left in surgeries and laser works as a point of increase of radius according to the drawing. The increase in radius increases proportionally the tension, since the venous pressure in the femoral is constant.
Laplace’s law P = 2T / r
Where P = pressure, T = voltage and r = radius.
So if we leave a saphenous stump larger than the femoral, the trend is growth. In the image a stump and some sizes for practical example.