How did giant sauropods satisfy their daily energy needs, considering their relatively small mouths?
Unlike mammals, saurischian dinosaurs don't have to chew their food. Like birds they have a gizzard to do that instead. 

Look at a modern ostriches: they have a very small head because the head is only used to gather food not process it. Mammals process food in the head (chewing) so the head has to be large to support the muscles, teeth, and other apparatus for this. On the other hand, birds and sauropods use stones in the gizzard to grind up their food, so all that space is taken up in the torso instead of the head.  Ornithischian dinosaurs, which do chew, have large heads just like mammals.

Additionally since the mouth is not occupied chewing the time that would be lost chewing can be spent feeding instead. This can be a lot of time a cow spends 8 hours a day just chewing. 

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