[tewall]

General tournament theory states that chips are worth more as the stack sizes go down, so if anything ICM should be undervaluing the true equity here, not overvaluing it.

Assuming the big stack knows how to use it to advantage, I'd assume the missing equity comes from the average stacks. This makes sense because the average stacks are the easiest to bully; big stacks have to respect other big stacks, and small stacks have pot odds to call anything, so the average ones are the ones susceptible to being stolen from, and hence the likely ones to supply the "missing" equity.