Why exactly would the Dinosaurs not have been able to support their own body weight? A skyscraper does not collapse under it's own weight. This is because it's structurally sound. Large dinosaurs had large, dense bones, and presumably had the strong muscles to go along with such large bones, otherwise they would have been crushed under their own weight regardless of the gravitational force exerted upon them. If something lives in an environment, it must be because it has evolved/adapted the that environment over time. This means that the Dinosaurs could, and in fact did, support their own body weight under the ~9.8m/s2 of force exerted upon them throughout their lives. There would have been no proportional difference between a dinosaur then, and a bird today. Each can support their own weight under the gravitational force. If they could not have, they would not have grown so large.