Actually, "let's build a cheaper one and hope it works out" is the design philosophy here, and it's a very effective one across pretty much all domains. The fact that it's bigger too is mostly incidental to its economic case. You think we would have stopped at Apollo 17 if the same Saturn V was capable of flying Apollo 18 - 30?

Hadn't we already built the Saturn V for Apollo 18, 19, and 20?