Why dinosaurs are not found today?

Assume that the big meteorite collide the earth 65 million years ago and declined the big dinosaurs. Why the remained small reptiles couldn¿t get bigger again. The earliest reptiles which were amphibious were very small but they could get bigger and made the dinosaurs. Then it becomes clear that the factor which caused them to extinction is still remained and doesn¿t allow today reptiles to get bigger. So the collision of the big meteorite with earth and the formation of dust can not be supposed as the factor of dinosaurs decline because that collision occurred at that time and the effect of dust and smoke have been disappeared and there was no later effect on animals. The theory of the increasing of the gravity says by increasing of the gravity the ability of blood system of these animals decreased gradually. So animals which had weaker blood system become smaller or extinct. Scientists think that the decline factor which was the big meteorite collided at the time and everything is finished and they can form new giant animals. These scientist are unaware that they don't as far the gravity of earth is like the gravity this time it is impossible to revival such giant animals.