I think the reason that people think that the small jellyfish are scary is because they look so harmless. I mean, think about it. They don't have eyes, teeth, a mouth or anything, so you would assume without knowing anything else about the animal that it wouldn't be able to do much to you. Then the experience of getting stung by a jellyfish very directly contradicts that notion, making it even more scary.
but yes scary! I mean I can;t go in the ocean because I'm scared of them and some you ca't even see! 
and the fact that some are toxic