How does this whole gasoline - greenhouse gases thing work?

I am seriously confused! I live in "Schwarzeneggarville" and just watched a story on the news in which they were talking about his Hummers and how much greenhouse gases they give off. They said that he uses 415 gallons of gas per year in one of his Hummers and that this results in 8,125 lbs of greenhouse emissions. I believe a liquid gallon of gasoline weighs about the same as a gallon of water, or 8.14 lbs. 8.14 x 415 = 3,378 lbs of gasoline used per year. How does 3,378 lbs of a liquid that is heavier than air become 8,125 lbs of lighter than air gas? This sounds like "fuzzy math". What else, on earth, can one burn and have it more than double in weight?