So Oasis sell 250,000 as a Knebworth one off (I think that was over 2 shows how many of those fans went to see Prodigy, Charlatans or Manic Street Preachers on the bill?) we average 25,000. What would Oasis have averaged performing 21 times over 9 months at the same venue, with the same again the next year? So it is opinion as we have no idea how many fans Oasis had or City for that matter its subjective.
Its silly to state as "fact" that one is 20 times bigger than the other for that reason, there is nothing to even vaguely rationalise the point either way other than your opinion which you are of course entitled to.