There needs to be a good balance of size, attitude and appetite, IMO, to be considered a monster. My biggest fish are pacus, but they aren't half the monster that my 6 inch GT is.
There are a lot of areas that a fish can be a monster. It can be a monster regarding size, attitude, appetite, predatory behaviour, temperrment or how they are ranked in their own ecosystem. There are a lot of small fish that are the top of the pyramid in their ecosystem.