Hi Other Tink,
Well I found some information on (that great medical pundit) Wikipedia, plus the Mayo Clinic site...
The enlarged testicular vein, I think that is what these sites call VARICOCOELE, and it occurs in about 15% of males anyway! Varicocoele is similar to varicose veins in other parts of the body. The cause is unknown, but these sites suggests a) genetic malformation, or b) flaw in the venous valves that results in the blood backing up, and damaging the venous wall.
As for the increasing overweight, well, I go along with Marianne's idea of encroaching Western diet, too many MacDonald Happy Meals...