The dinosaurs' large size has presented palaeontologists with an interesting paradox. Calculations of the structural dynamic stresses within the bones of the largest dinosaurs indicate that they were too large to move fast without injuring themselves. Contrary to this view is the fact that detailed bio-mechanical reconstructions indicates that they were agile, active creatures.
Depending on the arguments used, a particular dinosaur can be reconstructed in two ways, slow and lumbering, or fast and agile, with both sides of the argument appearing equally valid.
This paradox can be resolved by assuming that the dinosaurs lived in a reduced gravity. A reduced gravity would allow animals to still be highly mobile and agile at the gigantic sizes demonstrated by the larger dinosaurs because they weighed less.