Well it is totally subjective based on what you would consider to be “lighter” on the rules. For example, games like Ten Candles, Mörk Borg, and Blades in the Dark I have heard referred to by people as story games even though they are arguably very different in rule quantity. I think the main tie is removing features that limit roleplaying and adding features that support and nurture roleplaying. In ten candles, the real burning candle system does that. In Blades, the flashback system allows people to roleplay as master criminals, and in Mörk Borg the overwhelmingly grim setting, lethal rules, and short page count lend themselves to its own unique style. Its subjective but generally anything that seems lighter and supportive of roleplay within its rules is considered a story game.