A girl who is fat and doesn't even have big tits to compensate. Representing the shape of an actual snowball, basically just not attractive at all.
Snowballs are usually full of themselves, thinking they're the shit. A lot of them will wear push up bras to try to fit in when they are insecure about themselves.
All the perverts on this site have it wrong, it's when you blame something on someone so to unite your people against a common enemy, Stalin did it with America, Hitler did it with Jews, and Napoleon the pig, from George Orwell's masterpiece satire, Animal Farm, did it to Snowball, this is where the word originates.
Comrades, do you know who is responsible for this? Do you know the enemy who has come in the night and overthrown our windmill? SNOWBALL!!