Gigantic sauropod dinosaurs had relatively small heads (and consequently small mouths) in comparison to their overall body mass.
In the book "Dinosaur Odyssey: Fossil Threads in the Web of Life" (by Scott D. Sampson) the author says (on pp. 218-219) that
The sheer size of the biggest sauropods has long shrouded them in mystery. How could such gigantic animals with such small heads (at least compared to their body sizes) manage to consume enough food to sustain themselves? The mystery deepens when we look inside the mouths of sauropods. Cows are typical of big mammalian herbivores (e.g. elephants, deer, antelope, and the like) in possessing oversized teeth well suited for processing large amounts of fodder. Chewing serves not only to break food-items into bite-size chunks' it also grinds up the plant matter and begins the process of digestion by making the bits of vegetation more susceptible to chemical attack. By comparison, sauropods have puny teeth well suited to tearing or, at best, slicing, but not for grinding.
I don't remember Dr. Scott providing an explicit answer to the question "How could such gigantic animals with such small heads (at least compared to their body sizes) manage to consume enough food to sustain themselves?" but somewhere in the book he did mention that the earth's ecosystem may have been rather different back then.
However, since the book came out nearly a decade ago, I was curious on whether any other findings have been made since then.
How did sauropods manage to consume enough food to sustain themselves? Was it because of some differences in the ecosystem that are now lost?