I saw a plasticized corpse of a giraffe at a Body Worlds exhibit here and the scale of the innards of even that relatively small creature was amazing. The diaphragm looked like a small trampoline.
It's easy to get the food you need in a world where everything is as big as a car.
But you're right. In the modern world, where things are much smaller, a dinosaur-sized creature probably could not survive outside of the water without an extremely low metabolic rate.