Sorry, I think my phrasing might have been kind of weird - I was referring to the weights of H2 and N2 relative to CO2, which weighs a whopping 44 grams per mol.
…Although, I just did some quick estimates last night, and “almost twice as heavy” was still pretty far off. CO2 is much closer to 1.5x the weight of N2 than double the weight of N2.