It’s very basic organic chemistry. Each carbon atom from the fuel combines with two oxygen atoms from the air meaning the resulting waste product (CO2) is much heavier than the fuel. Likewise the very light hydrogen in the fuel combines with heavier oxygen at a ratio of 2:1 making water which is also heavier than the original fuel.
2 C12H26 + 37 O2 = 24 CO2 + 26 H2O
That brought on a cold shudder!