The SPX index is essentially a paper traded index and represents the aggregate inflow or outflow of money into the equities system. Size is irrelevant. I do find it impressive that index funds like vanguard can mimic the SPX so closely (especially when you consider they have to manage redemptions and inflows) but that is an operational problem rather than a stock picking one.
Again an operational problem that is managed by the streets’ program trading desks. Apples to oranges of citadels performance.