Why did dinosaurs grow to giant sizes, while mammals remained relatively small?

benonemusic writes: A new study in the journal PLOS ONE suggests that dinosaurs reached gigantic proportions relative to mammals because of differences in their cartilage, making their joints squishier and able to sustain greater amounts of force. Other factors contributed to dinosaurs' larger sizes, including their lighter, air-sac-filled skeletons, and some researchers point out that the sizes of some dinosaurs and mammals were approximately equal, so anatomical differences between cartilage in dinosaurs and mammals may not directly explain why some dinosaurs grew to larger sizes.