Hello there, I realise you have had your answer directly, so whatever I say is sort of second rate, but thought I would pick up the thread, as this question interests me also.
I have a very practical exercise to suggest: try to find as many photographs of yourself as a young child – two or three perhaps – any age really – from your family albums.
Observe carefully the size of the diaphragm and also the belly. Also note the muscular tone and elasticity of the lower abdominal muscles. If you have a young child, or know anyone with a young child, then follow up with direct observation.
After all this, remembering that the dantien is an energetic centre, you will have the answers.
Kind regards
Guy