The vast majority of the leakage is around the bead. The point of leakage is so big that it makes no difference as to whether your using pure nitrogen or 80%. Both will leak just as much because of the size of the leak.
The leaking through the tire story is marketing hype used by tire dealers to up-charge for Nitrogen. It's just marketing and they are trained to sell this way by the supplier.
Also nitrogen is not a lot bigger than 02. It is infinitesimally larger (75PM vs 72PM) which for the subject at hand is irrelevant.
It's only smaller because it's basically tighter because of charge.
