The extra weight comes from the OXYGEN (if you said "AIR" that’s OK too)! Oxygen (from the air) combines with carbon (in the fuel) to make CO2.

Details: Two atoms of oxygen (atomic weight 16) displace two atoms of hydrogen (atomic weight 1) to combine with each atom of carbon (atomic weight 12).

Before it’s burned, one sub-unit of fuel (CH2) has an atomic weight of 14.
(C=12, H=1; 12+1+1=14)

Burning (combustion) causes the carbon to combine with oxygen to form CO2, which has an atomic weight of 44.
(C=12, O=16; 12+16+16=44)

(The hydrogen also combines with oxygen, to form H2O (water), which has an atomic weight of 18. (H=1, O=16; 1+1+16=18) ... but that’s not part of the original question.)

That’s how fuel with a sub-unit atomic weight of around 14 can produce CO2 with an atomic weight of 44 or, as the question said it, how a gallon of fuel that weighs around 6.5 pounds can produce about 20 pounds of CO2 when it's burned!