The title has so many interpretations

It could be referring to the robot BayMax, since he is BIG.  Or it could be referring to Hiro, the... uhmmm...  hero.  Or it could be referring to the group of superheroes that circumstances bring together in the film.  Or it could be referring to...  well, I guess that's it.  If you can think of another one, just write a comment and include it.  Gotta be clean though!

Movie Review: Big Hero 6

Source: IMDb.com