Michael C. LaBarbera, a professor in Organismal Biology & Anatomy at the University of Chicago, published this wonderful paper about the reality of movie-monster anatomy in 2003. In the paper, LaBarbera explores the implications of extremely large and extremely small fantasy creatures, whose mass, volume and surface-area scale at different rates as they are shrunk/enlarged (ants can carry many times their body-weight, but if they were the size of tigers, they'd be crushed under their own carapaces). Other issues covered include the respiratory difficulties of Mothra, the biomechanics of Jurassic Park dinos, and the reason ET is so effing cute:
The upshot of all this is that Mothra is going to have to add a lot of tracheal tubes to maintain a sufficient oxygen supply. Of course, the more of its volume that is tracheal tubes, the less is biomass that needs oxygen, but this implies that although Mothra may be heavy (because it's big), its density is going to be very low–about the same as your average cotton ball.
This insight into Mothra's physiology eliminates two other problems. Although wearing one's skeleton on the outside has distinct mechanical advantages (as we'll see shortly), large insects are prone to a mode of failure called buckling. If Mothra had really been just a scaled-up moth, its legs would have collapsed when it landed. Second, Mothra's wings are in the same proportion to its body as the moths that bat their heads against the lights outside your door. Total lift generation is proportional to the area of the wings; if mass increased in proportion to volume, Mothra would have to walk home.
(Thanks, Glen!)
