If you remember back to taking chemistry in school, you might recall Avogadro's Law, which states that equal volumes of gases have the same number of molecules.

If you remember back to taking chemistry in school, you might recall Avogadro's Law, which states that equal volumes of gases have the same number of molecules. So if we're constantly breathing in a certain volume of gas and then breathing out that same volume of gas, then it would stand to reason that we have to be losing weight continuously, because CO2 has a higher molar mass than O2.