LiveScience (July 6, 2009)
"The secret to mega-dinosaurs' impressive sizes may be that the reptiles used more of their energy for growing and less for keeping their bodies warm compared with some creatures.
"A new model could help explain how some dinosaurs, such as long-necked sauropods, could have achieved masses of around 60 tons — about eight times the mass of an African elephant, the largest land animal alive today.
"The two main factors that determine vertebrate size are the amount of available food and how the creature expends its energy, said researcher Brian K. McNab, a paleontologist at the University of Florida. For example, elephants can be quite large because they feed off grasses, a relatively abundant food supply as opposed to say, the nectar that hummingbirds and bees consume, McNab said...."
The LiveScience article seems to be a pretty good overview of McNab's speculations, and a review, at least, of what we know - and have surmised - about dinosaurs.
I'm not sure if LiveScience over-simplified NcNab's work, or if this really is a fair sample:
"...'I think it was impossible for [dinosaurs] to have really high metabolic rates like mammals and birds, simply because the resources weren't there,' he told LiveScience.True: grasses didn't exist when these enormous animals did. But without a detailed analysis of the biome those dinosaurs lived in - including data on the caloric and other nutritional values of the plants available - I'm not sure that no grass = no big animals idea is really valid.
"For example, there were no grasses in the Mesozoic, which are a major food source for herbivores, McNab said...." (LiveScience)
But I'm no expert.
The point about larger animals shedding heat less rapidly than smaller ones has been made fairly often in discussions of dinosaur metabolism.
NcNab's speculations are another reason why I think researchers could learn a great deal, if it was possible to grow a living dinosaur. That's an idea that's getting less wildly improbable by the year: "Bringing Back the Dinosaurs: Not a Crazy Idea Any More" (June 29, 2009).