I've been surfing through Raymond Pierrehumbert's great book, Principles of Planetary Climate, especially Chapter 8 on the evolution of planetary climates. It turns out that getting smacked by big rocks can knock the heck out of an atmosphere.
Impactors which hit the Earth are moving so much faster than the speed of sound that the air in front of them literally has no time to get out of the way, and all piles up in front of them until enough momentum is transferred to that lump of compressed air to slow down the impactor. For Earth and its atmosphere, if the radius of meteorite is larger than a few meters, that won't happen before the impactor hits the ground, though slightly larger ones may explode in midair due to the pressure of the compression. Substantially larger impactors are scarcely slowed down at all before impact, and they blow a cone of atmosphere out into interplanetary space. Larger masses expand the angle of the cone. When the mass is large enough (the critical mass, corresponding to about 3-5 km radius, depending on composition), the cone expands to become all the atmosphere in the plane tangent to the point of impact. For Earth, such a tangent plane contains several ten-thousandths of the Earth's total atmosphere.