This article is from the Win-Spring 2018 AFRMA Rat & Mouse Tales news-magazine.
TThe amount of fat an animal stores is dependent on its being able to keep away from predators vs. starving to death. Storing fat will help an animal survive without food but slows their ability to escape a predator where those with lean muscle would starve more quickly. Also, the amount of fat an animal carries increases the energetic costs of movement. Medium-sized rodents carry the most fat. Other factors that determine how much an animal carries in fat are the sex, the amount of competition for food, and climate.
Some Animals Are Fatter Than Others on Seeker (was Discovery).