The heavy weight must have caused large stress on the tissue of the animals. There is a reason why these animals do not exist today, they would simply have been extremely slow and prone to injuries. So how could they exist at that time? The tissue must have been the same so this can not give a weight reduction. The only possibility is that the earth gravity was lower, so they did not weight so much. Compering a Tyrannosaurus Rex and an Elephant the reduction must be the half. This again indicates that the earth was half as massive as today and consequently a little smaller.
