The carbon comes from the coal and the oxygen from the air to make the carbon dioxide. As two of the three atoms to make the carbon dioxide are from the air and not the coal, it’s perfectly reasonable that more carbon dioxide would be released than the weight of the coal.
Oh...yep...and Oxygen has a higher atomic weight. I think I had my brain switched off there. Thanks...
I second Our man in washington’s answer.
It’s Stoichiometry 101.