You don't know anything about working out at all. You can fatigue with movements with no weights. Try putting your hands over your head for 1 min.
Light weights are good for definition of the muscles and cardio, not large weights. I currently am working on my cardio. Although I am physically stronger than many people in my circuit training gym, those people can still lift the same or more than me in a workout because they have better cardio and their muscles don't fatigue as easily. I personally would rather have the cardio than be the strongest.