It is all about the total energy transferred to the atmosphere and how fast. Just like your car, you can stop it basically instantly from 100 mph by hitting a solid mountain cliff and it will cause great destruction to the car or you can disapate that same energy with the little bitty brake pads against the rotors more slowly and nothing gets harmed except some very tiny amount of pad and rotor wear and some heat released into the atmosphere, the same amount of energy is dissipated either way. If the rock hits the earth directly at 20 miles per second it will make a much larger explosion than the same size mass traveling 1/2 the speed and gently easing into the atmosphere at an angle. The way stuff is just flying around in the galaxy something very small could hit the earth and cause major destruction. They say the earth is flying around at thousands of miles per hour relative to the sun, something very small traveling equally fast in the opposite direction could hit with some serious energy and would be much harder to detect ahead of time than a large object traveling relatively slow with the same amount of energy.