Modern Dinosaurs - Season 1 Episode 5 Island Giants

In New Zealand, a spikey grasshopper-like creature, the Giant Weta, can grow as big as a rat. These islands are home to some of the biggest birds ever known, giant centipedes and carnivorous snails, and a two metre long lake monster capable of slithering its way across beaches – the gigantic longfin eel. What advantage would animals have to grow so large? And why did Australia become a land of giant marsupials, while New Zealand developed giant birds, insects, and fish?