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.

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.