Legally, the bigger ones are. The real difference is (a) in the complexity and (b) the advantage in protection (and sometimes storage) a scooter gives versus the advantage in maneuverability a motorcycle gives...

They still need a helmet, and they still have you out in the breeze, and if that's what you're after, then modulo being careful, go for it. (I'm actually still considering a scooter myself kinda sorta... jenkitty has said I can't have an actual motorcycle, though, and I don't blame her.