The opening may be wider vertically and not horizontally wider. What do you do when you play a soft c below the stave (staff)? The air is coming out through a smaller aperture on the pp C, and then you crescendo by allowing more air to naturally make the aperture open up. If our lips are like vocal cords or guitar strings (I feel lips on a mouthpiece are NOT exactly like strings or cords), the vibrating material must have a greater amplitude. Watch a plucked guitar string. You can see the vibrating string Has a wide vibration. As the vibrating string loses energy, its vibration is much smaller. The string tension stays the same in both situations and so does the pitch. If you remember grade school general science, louder vibrations have a larger displacement from the resting position, but their frequency remains the same.

If you wait before playing a note so that you have less air, eventually you will have no choice but to play softer while using passive reduction. I believe you should be able to play softly using passive reduction on a full tank of air. I am convinced that I can. I believe Greg demonstrates that he can. I believe that you can too.

Now forget my aperture hypothesis and just answer these questions?
Can you play a soft note using passive reduction?
Can you play a medium loud note using passive reduction?
Con you play a very loud note using passive reduction?

Could Greg match your volume playing a note with passive reduction?
Could you match Greg’s volume playing a note with passive reduction?

Try this:
1. Take a concert breath. Let the air out passively in a sigh.
2. Take a concert breath. Let the air out through lips pursed and the teeth closer together than the sigh. Do you sustain a longer exhalation of air?
Repeat the second situation, but arch the tongue so there is very little space in your mouth. I think you should be able to sustain an even longer exhalation.
This was just to show you how air passively leaves our bodies can be controlled. I am not suggesting that we use any of these methods to control the loudness of a note.

So how do we control how much air is released, and thus control the volume?

Keep exploring, Francis, and you may solve what is happening. My thoughts are just that and my thoughts can be wrong.