Reference, homage, satire, archetype... You have tons of possibilities usually defined by the laws of whatever country you live in so plagiarism can be properly identified and punished. Plagiarism requires passing a particular, identifiable and often elaborate idea as your own with intent of hiding it. It's obvious Oda is just making references when he designs baron Tamago after Hercule Poirot, for example, which is way different than copying the plot of Murder on the Orient Express.
In this case, "small but smart" isn't even an author's personal and complex creation, it's an archetype —a shared matrix. So again, go cry for how Molière plagiarized The Miser, how Frankenstein wasn't an original idea, how every monkey-like Eastern hero comes from Journey to the West and so on.