Why do tubes filled with CO2 go soft so soon?

The rate of loss

Dear Lennard,
Since a CO2 molecule is larger than either an oxygen or nitrogen molecule, why does it leak out of a bicycle tire faster?
Glenn

Dear Glenn,
Upon receiving your question, I put CO2 in a clincher tire with a Michelin butyl inner tube (latex tubes leak air quickly, as you’re probably aware). This particular tire and tube hold air pressure faithfully for weeks on one of my road bikes without needing pumping. And sure enough, within three days after inflating with CO2 to 90psi, the pressure had dropped to 45ps
...
Permeation by diffusion predicts gas leakage rates proportional to the inverse of the square root of their molecular weights. Using air as a reference the predicted leakage rates for common gases are: helium 2.7, air 1.0, nitrogen 1.02, oxygen 0.95, argon 0.85, carbon dioxide 0.81.

It turns out however that the leakage rate of CO2 is huge, and the reason is that it is actually soluble in butyl rubber and is thus not constrained to normal permeation loss, it can transfer straight through the bulk rubber resulting in severe tire pressure loss on the order of a single day.

<a href="http://www.velonews.com/article/87175/technical-qa-with-lennard-zinn---large-molecules-and-short">http://www.velonews.com/article/87175/technical-qa-with-lennard-zinn---large-molecules-and-short</a>;