@lovelivelaugh I beleive it's just the body types, people who find it easy to stay lean are catagorised as ectomorph, they are mostly tall, skinny and lean. They find it harder to put on weight and commonly have high metobolisms.
Then there's endomorphs, these are people who typically find it easier to put on weight, to stay in shape they have to watch what they eat and train cardio more often. They don't find it as hard as ectomorphs to put on muscle.
Finally there is mesomorph, people built for putting on muscle. Some mesomorphs don't even have to pick up weights to put on muscle, they just build it really easily, when they do end up picking up weights, they'll look bulky and good.
You can be a mixture of these body types, typically you'll know which one you fit into. As for the people you linked that stay lean all the time, I'd say they must be ectomorphs, or maybe ectomorphs mixed with mesomorphs.