This is a great question. I had exactly the same thought some time ago and was scratching my head where the extra mass was coming from. Short answer, it pulls

This is a great question. I had exactly the same thought some time ago and was scratching my head where the extra mass was coming from. Short answer, it pulls oxygen from the air to make the CO2. Most of the mass of CO2 is from the O2 that came from the air, not the tank.