What is the significance of our final view of the squid/whale?

The squid and the whale is referenced at a key point in the movie when the son, Walt, opens up to his counselor about his go-to happy memory is with his mom—whom to this point he'd shown nothing but distain—a museum visit of a squid and whale display. In the movie's final scene, Walt goes to revisit that display. As he passes under the blue whale (which feeds on plankton and squid), we expect to see a tiny squid model. Instead, he passes that model to see the model of a giant squid attacking a big sperm whale.

This was kind of a reversal of expectation. I suppose it reflected our own observation of how Walt's feelings weren't what we expected; but I'm wondering whether it was something deeper than that? (Or perhaps something else, altogether?) What was the significance of the final scene (and, therefore, the title)?