That's what the article is all about. It's temperature management. Smaller warm-blooded animals have less mass to generate heat and higher surface area to volume ratio to dissipate it faster. They need a much higher metabolic rate to maintain body temperature, especially when the environment is very cold. They would freeze if they had the same rate as the elephant.

Large warm-blooded animals have more mass generating more heat and less surface area to dissipate it. They need a much lower rate or they would overheat. Not to mention the objectively huge amount of food and air circulation that would be required to maintain that rate.

If either animal had the other's metabolic rate, it would be dead within hours, if not minutes.