Man consumes about one fiftieth of his own weight in food daily, but a mouse will eat half its weight its own weight in a day. Thus, per pound of body weight, mice eat 25 times as much food as people, and yet they don't seem to grow any bigger. So where does it all go?

First of all, I find it EXTREMELY hard to believe that a human being consumes 1/50 of its own body weight per day. This means that a 200 pound man would consume 4 pounds of food per day. Secondly, a mouse's metabolism is much, much higher than a human's, which contributes to the fact that a mouse does not really grow that much bigger in proportion to the amount of food it consumes. It is also (to a lesser extent) dependent on how many calories are in the food, not necessarily the amount you eat. For example, you could eat celery all day long and never gain an ounce. However, eating a slice of chocolate cake could definitely lead to wider hips and a bigger gut.

As far as I am aware this only applies to warm blooded animals.
It is all down to the ratio of the animals surface area to its volume.
The smaller something is the higher the surface area compared to volume.
The larger an animal becomes, the volume increases significantly faster than its surface area. Warm blooded animals need to maintain their body heat, the smaller you are the harder it is. In fact it is impossible for a bird to exist smaller than a humming bird since it could not consume food fast enough. The smallest a mammal can be is something like a shrew.

To compare rates at which different animals use energy, scientists calculate for each the rate at which a resting animal under no stress consumes oxygen. That consumption is then used to calculate the basal metabolic rate, which is expressed as the number of kilocalories of energy used per kilogram of body weight, per hour. Small birds have proportionately larger surfaces (through which heat is lost) in relation to their mass of metabolizing tissue than do large birds. A Bushtit can maintain a body temperature like a Tundra Swan's because it has such a higher basal metabolism (i.e., uses proportionately more energy). Hummingbirds, with their tiny bodies and high levels of activity, have the highest metabolic rates of any animals -- roughly a dozen times that of a pigeon and a hundred times that of an elephant. To maintain those rates, hummers have to consume about their weight in nectar daily. In fact, a warm-blooded animal can't be smaller than a hummer or a shrew. Further reduction in size would make it impossible for the creature to eat fast enough to maintain its body temperature.

http://www.stanfordalumni.org/birdsite/text/essays/Metabolism.html