A tit bully is someone who's breasts are large, flaunted and outright bully others around them with smaller breasts.
In many instances flaunting is not required to be a Tit Bully. If it is obvious that someone has huge boobs then they are still considered a Tit Bully.
An example of a Tit Bully would be a girl with huge boobs. She in essence bully's those with smaller boobs.