How large is a CO2 moleucule?
This paper implies that masks trap them.
A molecule is incredibly small, of course, but there are a LOT of them in an exhaled breath. I suspect that despite their tiny size, the sheer volume of gas they make up creates back pressure, which is why enough CO2 is re-inhaled to cause a problem. The best estimate I can find for the number of CO2 molecules in an exhaled breath is 5.5 X 10 to the 20th. That’s 5,500,000,000,000,000,000,000 molecules in one exhalation. I guarantee there are nowhere near that number of virus particles in one breath.