Originally Posted by whiteminiman
It's physically larger than the other atoms that make up air, and so leaks out far less. I'm told that oxygen leaks out three times faster than nitrogen.
Nitrogen atoms are marginally larger than oxygen atoms (although there isn't much in it). However the air is made up from Nitrogen and oxygen molecules
not atoms (plus water, co2 and traces of other stuff!). A nitrogen molecule (N2) is smaller
than an oxygen molecule (O2) - largely due to the relative strength/length of N-N and O-O bonds.
If your hypothesis were true then the N2/O2 ratio of a tube/tyre filled with air would change over time. In effect if you inflated the tyre/tube until it started to go flat then topped it up - and repeated it would eventually end up with so much nitrogen that it wouldn't go flat any more - which is not the case.
The theory about water vapour seems much more logical.