Size and Cells
Animals can vary enormously in size, but they're alike in at least one way. The individual cells that compose all of their bodies--from shrews to people to dinosaurs--are roughly the same size. Big animals just have many more cells than little animals. That's important, because keeping those cells alive and functioning is the reason we animals eat and drink and breathe.
The six-inch (15-centimeter) Lilliputians in the novel Gulliver's Travels figured their human visitor would need 1,724 times their daily ration, assuming he burned energy at the same rate they did. But they were wrong: big animals use relatively less energy than small ones, so Gulliver would have needed only 267 times a Lilliputian's intake.
How Low Can You Go?
When an animal is very small, it has a lot of surface area relative to its volume--in other words, it has a lot of outside for its inside. This dwarf gecko, found in Cuba and the Dominican Republic, would never endure the direct sun as many elephants do; water loss through its skin would make it shrivel in no time. But by sticking to the most humid spots in its semidry home, and only coming out at night, the gecko can thrive.