Sauropod Gigantism. 2008. Science 322: 200-201.
From National Geographic news:
To outgrow their predators, sauropods didn't just need lots of food. They also needed to develop fast, so they could attain their full size before being eaten, experts said. Sauropod bones show that they did indeed grow swiftly. A 10kg hatchling could become a 100000Kg grown-up in about 20 to 30 years—quick by dinosaur time.
"This tells us that they must have been warm-blooded and had a high metabolic rate compared to cold-blooded creatures," said the University of Bonn's Sander.
So why don't we see gigantic elephants and crocodiles roaming around today? Experts think that reptiles, such as crocodiles, still maintain the egg-laying advantage, but their cold blood prevents them from growing fast enough to reach a great size. Mammals have warm blood, but can't grow as big as sauropods due to their slow reproductive strategy and the need to chew their food.