I disagree with you, not because a fish have certain attitude nor their size affect their status of earning the place of being a monster.
Its depend on whether you seen the fish before or not.
If the person think it is.. then it is..
Just like Frankestein, hes above average in size, but hes nice, so how come people still call him a monster?
Because he looks weird? one of a kind? and people never seen him before?
Just like if you haven't seen a neon tetra before, and now you see one. You might think its weird with all those neon color. Then you might say its a weird fish or weird little monster, cause of the bright coloration on the body