Aren't CO2 molecules smaller than oxygen molecules?
I don't think so. Oxygen is 2 oxygen atoms. CO2 adds a carbon to the 2 oxygen. Maybe there is something about the shape of the molecules/shape of the tube molecules that does it?

I've seen other people report that the CO2 leaks faster than dumb old air, but nobody can explain why.

Hmmm :thinking harder: Air is mostly nitrogen. Maybe the nitrogen molecules are bigger than the CO2 molecules?

I'll have to ask my daughter - the budding chemist.