Why when my feet & legs swell up my penis & balls is swollen?

Scrotal anasarca: In all likelihood, the condition that leads to the swelling of your feet and ankles is also causing the swelling in your scrotal skin. Excess fluid often accumulates beneath the skin of the scrotum and along the shaft of the penis in conditions such as congestive heart failure. Usually whatever measures are taken to reduce the fluid in your lower extremities will also be effective for reducing the swelling in the scrotum.

Common: Whatever clinical conditions cause retention of fluid or edema, and there are many (e.g. Congestive heart failure, renal or liver disease, low albumin, certain medications, etc) the most dependent areas of the body are most commonly affected which are the legs when standing or sitting and the male genitalia when sitting or supine. Treating cause and keeping them elevated will help prevent/treat.