Size and Scale
Size is much more than just a matter of appearances. It affects nearly everything an animal does. Big animals eat more than small animals; small animals breathe faster than big animals. Big animals generally live much longer than small ones.
Most small animals produce lots of offspring; most big animals, only a few. In other words, size matters; and nowhere does it matter more than in sauropod dinosaurs, the biggest land animals ever.
Are you stronger than an ant? Trick question. Of course you are, in absolute terms. But an ant can lift 100 times its weight, and--unless you're a circus strongman--hoisting anything that weighs more than about 70 percent of yourself is a big challenge. In a relative sense, the ant is stronger. In general, strength increases with size, but not in a one-to-one ratio. Scientists say strength doesn't scale up with size.
Animals can vary enormously in size, but they're alike in at least one way. The individual cells that compose all of their bodies--from shrews to people to dinosaurs--are roughly the same size. Big animals just have many more cells than little animals. That's important, because keeping those cells alive and functioning is the reason we animals eat and drink and breathe.
A long life means different things to creatures of different sizes and metabolic rates. For a shrew it may be one to three years; for an elephant, 40; for a sauropod dinosaur, perhaps 60. Yet for reasons that experts are still debating, many living animals seem to have roughly the same number of heartbeats over a lifespan.
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.