The Trend Towards Big
The fossil record seems to show that many animal groups get bigger over evolutionary time. Even sauropods started off relatively small. What explains the size increase, when it happens? One factor may be that bigger is safer: predators think twice about targeting the biggest animal in a group.
Bucking the Trend
Not every group gets bigger forever. Modern birds--even the tiny bee hummingbird (Mellisuga helenae), the smallest living bird--belong to the dinosaur lineage. And the largest modern bird, the recently extinct 880-pound (400-kilogram) elephant bird (Aepyornis maximus) of Madagascar would have been dwarfed by the largest representatives of its ancestral dinosaur group.
Being big has advantages, but there are upper limits on size. Experts think the giant titan beetle, for instance, is about as big as a beetle can get. Why? In insects, the oxygen to keep cells alive isn't forcefully delivered by lungs and heart. Instead, it seeps through the body largely by diffusion. That passive method means not enough oxygen could get to the inside of a really big beetle.
There hasn't been a dragonfly the size of Meganeuropsis permiana for about 300 million years. Many experts think the high oxygen level in the Permian atmosphere (300-250 million years ago) allowed them to get so big. Even then, though, most insects were only an inch or so in size. Oxygen levels weren't high when sauropods lived, so that had nothing to do with sauropod gigantism.