> Big was the whole point

Big was political. It was a requirement added because SLS could provide it, not because it was needed. It was there to stop the order-of-magnitude cheaper per pound smaller launchers from being proposed instead.

Part of all this was strong arm twisting from the politicians involved to stop all work on in-space propellant transfer and propellant depots (because with them, you might as well launch lots of smaller cheaper launchers to carry up the propellant.)