After many generations, the airfoil shape we know today was set, giving avian dinosaurs the ability to quickly and safely transport themselves wherever they could thrive, an incredible evolutionary advantage. This is only one scenario. It is possible that something entirely different occurred. There are several possible reasons for this. The fossil record is biased toward large animals.
Large bones are more likely to survive to be fossilized than small bones.
If you were to count every bone laying on the ground on the continent of Africa, most of the bones you would find would be of large animals. Studies have been done to test this hypothesis, and indeed, in the test areas, nearly all the bones found were of large animals. The thousands of small animals were not represented by the bone clutter. Similarly, only a very small percentage of small animals are represented in the fossil record. Just as there are hundreds of species of small animals today, it is likely that there were tens or hundreds of thousands of species of small dinosaurs over the course of their reign.