It's one of those initially unintuitive things that makes perfect sense when you consider that "burning calories" is... pretty literally burning calories. Of course CO2 is how most of the mass goes away.
Similar to the "trees are mostly made from air" thing.
https://biologyinsights.com/how-much-carbon-dioxide-do-we-ex...
Ok, that is way more than the dozen grams per day I thought it was.