It has to do with the paleness of your skin and the thickness (or lack of thickness) of subcutaneous tissue (if you look at elderly people who have no "meat on their bones" you can see very prominent veins). Fat is, of course, one of the subcutaneous tissues, so as you lose fat, everything ends up SLIGHTLY closer to the surface. But mostly I think it's skin color - the lighter you are, the easier it is to see darker things (like veins). I have the same problem.