When President Obama did an interview on Reddit in August, one of the questions he got was:
Would you rather fight 100 duck-sized horses or one horse-sized duck?
Obama declined to answer the question, but Conor Friedersdorf decided to ask a waterfowl expert what he thought and the answer involves an interesting mix of science:
With such a huge body, the problem of surface area to body volume comes into play. The terror-ducktyl would have a problem losing heat. Hence, a possible tactic would be to get it running around chasing me and it might overheat, stroke out, and die. Birds have higher body temperatures than mammals in any case (often very close to the 40 degrees Celsius upper lethal limit) so it might not take too much to push the duck over the metabolic cliff. Merits consideration.
After engaging his graduate students in conversation, he came to realize that it would be politically disastrous for Obama to fight the duck-sized horses. Think about it. In America, the duck lobby is composed of duck hunters. The horse lobby is made up of horse lovers who succeeded in stopping Californians from buying horse meat. The young women voters essential to the Democratic coalition are far more sympathetic to veritable ponies than a giant, rape-obsessed mallard. Shooting the duck would be perfectly legal under existing law, or would at worst result in a citation for hunting without a license.
See also The Biology of B-Movie Monsters by Michael LaBarbera, a classic and one of my favorite internet things ever.
When the Incredible Shrinking Man stops shrinking, he is about an inch tall, down by a factor of about 70 in linear dimensions. Thus, the surface area of his body, through which he loses heat, has decreased by a factor of 70 x 70 or about 5,000 times, but the mass of his body, which generates the heat, has decreased by 70 x 70 x 70 or 350,000 times. He's clearly going to have a hard time maintaining his body temperature (even though his clothes are now conveniently shrinking with him) unless his metabolic rate increases drastically.
Luckily, his lung area has only decreased by 5,000-fold, so he can get the relatively larger supply of oxygen he needs, but he's going to have to supply his body with much more fuel; like a shrew, he'll probably have to eat his own weight daily just to stay alive. He'll also have to give up sleeping and eat 24 hours a day or risk starving before he wakes up in the morning (unless he can learn the trick used by hummingbirds of lowering their body temperatures while they sleep).