Better yet, even more on topic, plants were more nutritious back then before they micro-devolved into the crappy kind we have today. See! Now you can stuff 1.8 million cubic feet of hay into just a few thousand cubic feet of super-awesome-pre-flood-plant-food-stuff.
Would that be like the lambas bread in Lord of the Rings? So maybe Noah and his kin were elves and provided the food for all the animals in small, chock-full of nutrition loaves?