Lifting more weight isn't necessarily a precursor to increasing the size of your muscles. Not lifting enough weight, will, however, decrease or blunt your potential to successfully accomplishing your physique goals. If, for example, your goal is toning and firming, but you chose to stick with the same weights, reps and sets every time you train with weights, then you're quickly working toward a self-fulfilling prophecy of minimal improvement. If all you perform is 3 sets of 20 reps, using the same weight, then I would suffice to say that you're no longer pushing yourself to achieve new goals. You're simply going through the motions of exercise, hoping to get new results from the same old tactics. Increase the weight, and you're going to see better, faster results. Sure, you may not be able to complete the full 3 sets of 20 reps, at first, but you now have a goal. Over time, this weight will also become easy and it'll be time once again to change in order to see further improvements.