Chicago's stadium can hold roughly 20k (I think), while Seattle can hold 70k in their NFL stadium. That said, I can't say we would have filled up Chicago's NFL stadium if the game was held there.The support was good, but again a low attendance compared to Seattle. Maybe the Seattle figure was exceptional?
Seattle has a much larger soccer fan base. Although, Chicago is so central it was able to draw Spurs fans from all over the USA.