An Enigma

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Dieter Mei-Shin tends to eat slowly, masticating well. It looks good for health, yet might as well stimulate her stomach little by little to crave one more bite, over and over. She therefore takes too long a time to eat 80% of capacity. On the other hand, Shin and his mother eat so quickly that his wife still has half of her serving when they finish theirs up. Why one of them burly while the other skinny?