Hydrocarbons are mostly C by mass. Carbon dioxide is 27% C, 73% O (one C atom to two O atoms). So the output of one ton of hydrocarbons (coal, gasoline, paraffin, it doesn't matter) is three tons of CO2.

Some of the rest of the 14 parts air goes to burning the H atoms, which don't weigh much, and producing water. Most of it is nitrogen which isn't burned.

In an ideal burning reaction in atmospheric air for C4H10 you get 4 CO2 and 5 H2O (and a bunch of N2) at fuel ratio of 15:1.

So for 40 tons of fuel with C4H10 you’ll get around 120-130 tons of CO2.

Once you do the math and convert from mol for each product to KG it more or less comes out right. Hydrogen is 2 g/mol, Oxygen and Carbon are 16 and 12 respectively. So you are essentially tripling the mass simply because you are adding a heavy molecule (oxygen) to the mix.