Getting ripped requires striking a delicate balance between high lean muscle mass and low body fat. Lifting light weights, however, has little effect on either of those factors.
Lean Muscle Mass
Muscle size increases due to the effects of resistance exercise, which produces micro-tears in the muscle fibers. During the rest period that follows, the muscles repair and rebuild themselves larger than they were initially. Unless they’re lifted for sustained periods of time -- think 100 or more repetitions -- light weights do not offer the level of resistance required to produce this effect.
Low Body Fat
You can lose body fat by burning more calories than you consume. For two reasons, lifting light weights has little effect on the number of calories you burn. First, it does not require very much energy to accomplish and therefore burns few calories. Second, lifting light weights does not increase muscle size and therefore cannot increase your basal metabolic rate, which is the amount of calories you burn while you’re sleeping or sitting on the couch.