There is good evidence to suggest that the larger some one is (higher the BMI) the more likely they are to get varicose veins (Phlebology.2010;25:236).
This is despite the fact that normal superficial veins in a thin person may appear large, whereas varicose veins in an obese person may be hidden.
Hence, often when people loose a considerable amount of weight, perhaps following gastric banding operations, the leg varicose veins can become much more prominent. It is not the loosing weight that causes the veins
There are also studies which show that the complications of varicose veins are more likely to occur in obese people. The risk of developing chronic venous insufficiency skin changes at the ankle includingvenous leg ulcers is higher in over weight individuals (Biomed Pap Med Fac Univ Palacky Olomouc Czech Repub. 2011;155:367) as is the risk of developing superficial thrombophlebitis Eur J Vasc Endovasc Surg.2012 43:355-8 and Deep vein thrombosis.
Management of varicose veins in people who are over weight also has problems. Often support stockings are harder to get and keep on when someone is over weight. If surgery is considered, irrespective of the approach taken, the procedure takes longer, blood loss higher and bruising more extensive. The normally low risks of anesthesia (both local and general) are increased.