You are right in that it wouldn't of been that much extra, structurally, as the majority of the stadiums mass will be through the stands and therefore in piles through the ground external to what would be the underground CP. It would have a huge impact on services though, as all that toxic, fume filled air has to get out somewhere. A space of that size will need a much bigger path to escape than what can be afforded through a single entrance/exit. That would mean one of two things:
A) External exhaust/flues within the fan plaza space.
B) Vertical exhaust ducts throughout the structure to escape on the roof.
For A, it would have to be on the fan plaza, as N/S flues would have to puncture the existing dock structure to get there and any point of escape will likely cause congestion at those areas. The flues cant be low level as they're pumping out noxious fumes a their highest concentration at the very time people will be expected to be passing any external flue, so would be very visible.
For B, the exhaust flues will be highly concentrated, due to volume of fumes to be removed (everybody, all at once calls for a much higher air change rate than normal) and with passing though the structure, service risers will have to be created which would reduce internal space and therefore potentially capacity.