Comment by scotty79

It is directly proportional because of how lungs work. The percentage of CO2 in exhaled air is roughly the same regardless of how fast you are breathing (if you don't artificially accelerate your breathing beyond what your body dictates).

Lungs can't put in more CO2 in breathed out air.

If you breath only slightly faster when you are running than when you are sitting that must mean you ran quite a lot and optimized your stride so that you don't burn much more calories running than sitting.

But more likely explanation is that you are not aware how fast exactly you are breathing when sitting and running. Try to measure it. Also take into account that you might be taking shallower breaths when you are sitting.

Just blow up a balloon when you are sitting and when you are running with your normal exhaled breath for few seconds to test it.