Really, it's decreasing your body fat percentage. The less fat between your skin and your muscle, the more toned you will appear. Your muscles don't change appearance when you go low weight/high rep. If you took a flabby-looking powerlifter and did a full body liposuction job, he would look cut.High reps and lighter weight or heavy weight and lower reps?
If you are already a skinny dude, then you need to hit the weights.