A skinny man of 2m tall will be able to output significantly more power than a man of 1.5m tall who has the same fat percentage. Somewhere in between lies the perfect balance between power, weight and height. In the context of climbing you could assume the smaller the better, but both the tiny man and the taller man also have to carry the weight of the bike up the mountain, which in both cases has a minimum weight requirement. So if the tiny man weighs 45kg, for which his power output would be ideal, then you have to add to that another 10kg of bike, clothes, shoes, etc on top of his own weight, and suddenly his power/weight ratio isn't that great anymore. Percentage wise, this is less of an issue for the tall man who may weigh 65kg, as the added weight disturbs his power/weight ratio less. But when a taller guy can get his weight down to the weight of the smaller guy assuming both are still healthy, he will always be able to output more power.
All these guys are elite athletes. In order to improve, they are looking for details to give them just that bit of an extra push. Losing or adding 2kg of fat is NOT a detail. I assume next time you take your bike on a cycling vacation to France, you time your ascent of the Mont Ventoux or similar. First time you make sure you are carrying 2 liters of water on your back. Then two days later you go back, and do the same but this time without the 2 liters of water. Let us know how it went and how insignificant the extra weight was.