Some of the people with plumper builds absorb a bit more, but because they haven't practiced martial arts, or are unable to, the energy exists in other forms.
To put it in terms from his past life: fat.
Those who pursue martial arts can eat a lot, because they can convert and retain most of the energy from food and medicinal soups, then, in conjunction with cultivation techniques, refine that stored energy in the body into the much more useful Qi and Blood Power.
The essence of cultivation techniques is actually to help martial artists absorb and store energy, and to refine that energy into Qi and Blood Power.
As for the higher realms like Strong Bone and Refining Organ, those are about advanced utilization of Qi and Blood Power.
This was Chu Ming's summary from inferring Blood Circulation Realm techniques up to his current Refining Organ Realm technique.
