Seriously???. I'm pretty sure I'm not breathing in any more oxygen now, nor exhaling any more CO2 than I did when my weight was stuck at 265 pounds all those years. I AM however putting out a great deal more water! But that could be because I'm also drinking more fluids.

I personally suspect that at least some of the excess fat gets excreted as solid or semisolid matter rather than being completely converted into gaseous CO2.