Thanks for asking this question, had always meant to do so. Having always doubted the O2 bigger story in my mind dubious due to the MW being greater, have found and been corrected by (presuming it's true - sounds authentic):
www.getnitrogen.org/pdf/graham.pdf
I wouldn't think one expands more than another - physics hazy these days but I think the only time the ideal gas laws don't approximate reality is close to the liquid or sublimation points - certainly not relevant to O2 or N2 in these conditions