I think the key factor is not the size, but the type of management: Apollo, SpaceX (and the Manhattan project for that matter) all had technical leadership, and where headed by people who could basically have pulled the whole project off with a sufficient number of clones of themselves.

ITER is under MBA/Political management. Even if the management team had all the time in the world, they could not complete the project on their own.

Especially in ground-breaking projects, this makes a huge difference. With many options open, deciding on how to proceed requires cross-subsystem trade-offs and weighing many different risks and challenges against each other: things that will grind any non-technical leadership to a halt as they request piles upon piles of reports from their underlings.