How can CO2 weigh more than the gas it was derived from?

This is actually quite easy. The carbon in the gasoline combines with the oxygen in the air to form carbon dioxide. For each molecule of carbon (atomic weight = 12) you combine two molecules of oxygen (atomic weight 16) so you get a total atomic weight of 12+2*16 = 44. Thus, the carbon dioxide produced weights almost four times as the carbon burned.