Originally Posted by cyccommute

Yes, CO2 molecules are larger than both nitrogen and oxygen molecules (we don't really need to worry about the rest) both in mass and overall size. They don't move any faster than other gas molecules. In a given vessel all the gas molecules are moving at the same rate anyway. The reason that CO2 moves through the rubber of the tube probably has more to do with solubility of the gas in the rubber polymer than anything else. The rubber structure has pores in it which allow the gases inside to move out of the inner chamber. CO2 structure may fit the pore structure better than the other gases or it may actual form a solid/gas solution which allows the gas to exit faster than normal air. If you used a different material for the tube, say a polyurethane or a polyethylene, you would get a different diffusion rate. The problems with those polymers is that they aren't a cheap as rubber nor as elastic, i.e. they make sucky tubes.