There's a very long review out which presents a theory for how sauropod dinosaurs could scale up to such enormous sizes, Biology of the sauropod dinosaurs: the evolution of gigantism. ScienceDaily is promoting the likelihood that sauropods did not chew, and so could make do with very small heads which could be supported by long necks, as the big factor. But this is a model with many moving parts. Here's the verbal list from the conclusion:
(1) Sauropod dinosaurs as the largest terrestrial animals ever represent a challenge to evolutionary biologists trying to understand body size evolution. (2) The study of the upper limit of body size must address extrinsic as well as intrinsic factors, and it must be determined whether this limit is set by the bauplan of the organisms or by physical and ecological constraints imposed by the environment. Among several possible approaches, we chose the resource perspective because it has been shown that resource availability and maximal body size correlate closely (Burness et al., 2001). (3) In the interplay of the biology of sauropod dinosaurs with their environment, a unique combination of plesiomorphic features (i.e., inherited from their ancestors) and evolutionary novelties emerge as the key for a more efficient use of resources by sauropods than by other terrestrial herbivore lineages. Plesiomorphic features of sauropods were many small offspring, the lack of mastication and the lack of a gastric mill. The evolutionary innovations were an avian-style respiratory system and a high basal metabolic rate. (4) We posit that the long neck of sauropods was central to the energy-efficient food uptake of sauropods because it permitted food uptake over a large volume with a stationary body. (5) In the Late Triassic and Early Jurassic (210–175 million years ago), the combination of biological properties listed above led to an evolutionary cascade in the sauropodomorph lineage characterized by selection for ever larger body size, mainly driven by predation pressure from theropod dinosaurs. (6)From the Middle Jurassic onward, sauropod dinosaurs dominated global terrestrial ecosystems only to succumb to the catastrophic environmental change at the end of the Cretaceous 65 million years ago.
And here's a schematic illustrating the interplay of evolutionary forces & constraints:
The paper is open access, so you should read it yourself if you're interested.