The larger the rock and/or the faster the velocity, the larger the crater; no one says the rock was the same size each time.
http://www.hartrao.ac.za/other/tswaing/tswaing.html
What Made the Crater?
Drilling in the crater has shown that the rocks forming it have been shocked by an explosion, and are not volcanic in origin. In the explosion that formed the crater the incoming object would have been largely vaporised - there is no large buried object in or under the crater. The absence of such a buried object at the Barringer crater in Arizona caused confusion as to its origins for many years, although in that case traces of the original iron asteroid have been found.