I don't think you can grow new muscle fibers natural. But you can with GH, IGF and Steroids. Only thing you can do while being natural is with time increase your muscles ability to accept and hold more shit with training and diet. You simply get your muscle fibers to hold more fluids, glucose, sodium, creatine and other junk from continuous training and eating, hence they appear bigger, the muscles fibers stretch like the stomach stretch on an obese person. Basically just another form of overeating and obesity.