A few years back, there was a Straits Times article that revealed that different people have different capacity to metabolize fat. This is due to none other than hereditary factors. The liver has a fat-metabolising function which differs and its effectiveness in doing that differs in individuals.
Which means, two people can sit down and tuck in to a bowl of pig's trotters and roast pork... one will be ok after eating it, the other would have a liver that'll be working very hard afterwards.

Also, a person being fat or skinny is NOT a reflection of health... a skinny person could be walking around with a sky-high cholesterol level whilst the fat one may have normal cholesterol levels.
That's life!
