it's a game that is hyped up to matter (grrrrivals etc)
Much Bigger population to draw from
A lot of away fans travel to see those types of games that 'matter'. Probably puts the crowd over 20k
That particular fixture has had some awesome games in past seasons
Just to illustrate that, Sydney's highest attendance was 40k vs WSW. Rival game, plenty of travelling fans effectively doubling the crowd.
Their lowest was 11k vs Perth. That is only 3000 more than our average (that doesn't have those types of games - at least not until the end of the season if we are doing well) with a population that is much bigger than ours.