The impossible anatomy of Godzilla:
http://www.popularmechanics.com/technology/digital/fact-vs-fiction/the-impossible-anatomy-of-godzilla-16785535
Yep, same reason gigantic insects can't exist - environmental constraints. Here is an interesting study about - http://phys.org/news79804314.html
Basically, giant insects would require a higher oxygen composition in the atmosphere - this is why they were larger in the Dinosaur periods, as the oxygen content of the air was higher.
But of course a movie is a movie, and watching giant monsters wreck our cities and be completely unstoppable is awesome