Actually it was a planet size asteroid that fragmented off of earth and created the moon. The moon is earth material, because an object collided with it so big that it blew apart the earth literally and formed the moon. The biggest craters on the moon, like you said are about 180-200 miles in diameter. The Deccan Traps crater is 1,500 miles. If you dont think the impact factor of lava flows that extend twice the area of the UK are bigger than almost any asteriod that has hit this planet, you need to read up on supervolcanoes. The CO2, gases, etc from and explusion are by far a number of factors bigger than any impact event, except for a couple several billion years ago.
Yes, asteroids come in all sizes, but the fact is, we don't know how large the object was that almost split this tiny planet in two.
In addition, our scientist do not know how many super impacts have occurred in the past 4.6 billion years or so, due to the changing earths surface. They have only found what still exists, or what they have stumbled upon at this point.
Yellowstone is a large volcano, but it's certainly no planet killer, nor can any volcano match the cataclysmic energy released by a large asteroid or comet striking this planets surface.