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.

Couldn't smaller animals just evolve better insulation instead? Surely it would be easier to survive if they didn't have to eat so much.

Evolution doesn't pick the optimal solution, it just weeds out the solutions ones that aren't good enough.

Nature's insulation is fat and fur/feathers but there is a limit to how much that can help. It adds weight and bulk - hummingbirds have to hover their own weight so adding more fat is like making bigger rocket: you need more fuel to lift your fuel.

More insulation has other tradeoffs. Look at arctic animals - they're not very agile or dextrous. Penguins, whales, sealions, polar bears... they're all 'lumpy' and nowhere near as agile as other areas. Even the occasional agile animal like a snow fox is 'lumpier' than its temperate brethren, but they only have to be more agile than the tubes of fat that hunt them.

Any time you think "why didn't this or that feature evolve a different way", have a think about tradeoffs in terms of survivability.

The article claims "Without its internal heaters turned up to-the-max, a shrew would leak out all of its heat and freeze. And this explains its furious appetite."

If I was to hazard a guess, it's because the vole has a lot more competition for food. The elephant's main risk is eating up all the food and having none left. The vole's main risk is having some other animal grab the food first. At the vole's size, and with the vole's eating strategies, there's a real advantage to being quick. With the elephant and similarly sized animals, not really (and also, there's the exploding problem).

There's also the problem that a skeleton doesn't get stronger at the same rate that a body gets heavier, so a larger animal is going to have to avoid rapid accelerations in order to not break its bones or sprain its ligaments.

I think it relates to the surface area:volume ratio. The implication is that a smaller animal will have a relatively higher surface area to lose heat to, thus necessitating increased metabolic rate to make up for that loss.

http://www.tiem.utk.edu/~gross/bioed/bealsmodules/area_volum... provides some good basic info