Wired writes: Giants are difficult to make. Remember all those classic b-movies featuring giant insects, like THEM!, The Deadly Mantis, and Earth vs. the Spider? Mercifully for us, arthropods of city-destroying size could never exist. They would not be able to obtain enough oxygen to survive, and, without major modifications to their bodies, would collapse under their own weight. (There were giant arthropods over 300 million years ago, but even they were not of horror-movie size. Unless you’re deathly afraid of huge millipedes, anyway.) The same goes for vertebrates. The elephant-sized, bloodthirsty bunnies from Night of the Lepus would require changes in proportion to cope with the different stresses and strains that come with increased body size. There are constraints that limit how big organisms can get and what shape they can take as they get bigger.