If you put a straight edge on the table of your mouthpiece you will notice that at some point the mouthpiece begins to curve away from a straight line as you proceed towards the tip. When you blow on a mouthpiece you cause the reed to vibrate and the steeper the curve (the greater the tip opening) the more air that can be put into sound production before the tip of the reed contacts the tip of the mouthpiece. But obviously it will be harder to bend a stiff reed over a steeply curved facing; you have two choices. You can blow harder or you can use a more flexible reed. Try blowing on some different 'pieces with varied openings using the same reed and you will be able to feel the difference.