Jim is the narrator of My Ántonia and so the story is told entirely through his perspective. He is also the central character in that this is his story and not Ántonia's (just consider Book 3, in which Ántonia is absent). It is Jim who grows up in this coming-of-age novel, and we are primarily concerned with the way his character changes and grows over the course of the story.
On the other hand, some have argued that Ántonia, not Jim, is the protagonist of the novel. To start, she is the title character and the impetus for the fictional memoir Jim writes. We might follow Jim's story, but that story itself is centered around Ántonia. Even when she is physically absent, she's there in spirit. Jim can't stop thinking about her, and neither can the reader. Ántonia comes of age just as Jim does, which means she is as much a contender for central figure as he. She faces the hardships and struggles of being an immigrant in a new country, and the reader roots for her success – another hallmark of a typical protagonist.