On the city of ember movie why are bugs and animals so large?

The movie "City of Ember" doesn't depict bugs and animals as being unusually large. The film is based on the book of the same name by Jeanne DuPrau, and in both the book and the film, the creatures that inhabit the city are normal in size.

It's possible you are thinking of a different movie or fictional work. Can you provide more details about the movie you are thinking of?