The sauropod dinosaurs were able to grow to the largest land animals ever because they found a way to make their skeletons much lighter, using the same thing that birds do. Air sacs!

The air sacs have been shown to invade the bones, making them more lightweight. The aie circulatory system allowed them to shed heat more quickly which is a large problem for large animals. As well as take in more oxygen

(Disclaimer; since we don't have any lungs or actual air sacs from sauropods, we look at the shape of the bones to infer they were there by comparing them with modern reptile and bird bones for similar marks)

If I remember right, the giant ground sloths lived in an environment without too much more oxygen than we do today. Oxygen alone is not the only control factor for big size. The ground slots probably got big to help avoid predators, not unlike modern elephants.