My uneducated guess is that "small" and "big" refer to the conflict. Small means it isn't relevant to enough people, there aren't overarching themes that everyone can relate to, it's sort of... claustrophobic. Like, a book that deals with the experiences of a boy getting bullied, and nothing else. So "big" would be a story where the conflict is TOO... overarching. Or it has TOO many themes and conflicts going on. In that it feels bland, or else it makes the reader feel overwhelmed? Maybe? I'm more sure of my thoughts on Small than Big. hahaha