I've read several studies speculating on sauropoda gigantism. Unfortunately we don't have many fossils of nonavian guts, but we do have clear evidence from when digestion begins; the mouth. Below are the teeth of some of the largest mammals & one ofthe largest dinosaurs that's not a sauropoda. All share a diet of mostly greens.

The largest known elephant, paleoxodon n., weighed a conservative 16 tons, with upper estimates at 23 tons. The largest known hornless rhino, paraceratherium t., also weighed a conservative 16 tons with upper estimates at 23 tons. The largest known dinosaur that's not a sauropoda, shantungosaurus, also weighed a conservative 16 tons with upper estimates at 23 tons.

All estimates have variation based on study but there are similarities.

All began digestion with mastication. All had specialized molars/tooth battery to grind tough greens. Chewing is time consuming. We do know that the largest sauropoda, the Titanosaurs, had massive guts. Their teeth were also not for Chewing. The theory is that they didn't waist time Chewing but instead consumed as many calories as they could swallow while the gut did all of the work.
Thoughts?