I have added 3cm(about an inch) to my quad circumference naturally over the last 12 months. I have not added weight to my squat, hack squat or hamstring curls. I've changed every 10 weeks. Either by reps(sometimes up to 25 for squats) drop sets(I believe I responded best to this) speed of completed rep(slow controlled for the negative then changed for the positive). All while never increase in actually weight, I just don't feel comfortable adding extra weight on my back atm and utilised other methods of changing the intensity.