It's because the majority of the mass of CO2 comes from the oxygen from the air used to burn the carbon. So in this case, it's quite literally creating mass out of thin air.
Specifically, the atomic mass of carbon is 12 and oxygen is 16. So for CO2, 12/44 of the mass is carbon, or about 27%. So of that 20 pounds of CO2, about 5 1/2 pounds are carbon, the other 15 pounds is oxygen sucked out of the atmosphere.
