In 1954, Godzilla was 164 feet tall. With each successive appearance over the past 60 years, his height has increased, so that he now stands at 492 feet. Not bad, given that it took cetaceans 55 million years to go from 8.2 feet to 98 feet in length. Can evolutionary theory help explain Godzilla's rapid growth spurts?
Marine biologist Craig McClain—the Assistant Director of Science for the National Evolutionary Synthesis Center—has an explanation over at his blog, Deep Sea News. In a word, McClain's answer is "skyscrapers."
Skyscraper height has increased dramatically over the last century. For Godzilla to continue to plow through buildings in major metropolises, a more formidable size is needed. Of course, this size change can only be evolutionarily adaptive if it changes the fitness of Godzilla, i.e. in the simplest case the number of offspring passed to the next generation. If Godzilla is able to topple buildings this might allow for greater acquisition of resources—in this case, food in the form of people. This would increase the lifespan of Godzilla [and] allow for more reproduction or allow for greater amount of energy to be passed to the offspring, increasing their rate of survival. Or perhaps toppling buildings is a sexual display that sexual partners cue on. Sexual selection!
On a side note, McClain calculates that Godzilla would produce 151,436,928 gallons of urine per day. Take heed, Portland.