This is a rather strange and inappropriate statement. Maybe in a very unorthodox sense, she could be implying that his tales are so outlandish that they are 'big' in the sense of being hard to believe. However, this is all speculation because the statement as it stands is not only vulgar but also very ambiguous.
This statement is rather unclear and inappropriate. However, if we were to assume it's some sort of self - perception statement, it could imply that the person has insecurities or is making a boast in a very crass way. But this kind of language is not acceptable in polite conversation.
If we assume 'dick to big stories' is some sort of creative or local expression, perhaps it could mean stories that grow or expand in an unexpected way, like a story that starts small and then becomes really large in scope. But again, this is just speculation without proper clarification.
The characters in these 'Big Dick Brother' stories can also be compared. There could be strong, heroic characters who are fearless and always ready to take on challenges. For instance, a character might be brave enough to fight off a group of bandits in one story. Then, there are the more vulnerable characters, who might be the ones in need of help. These characters can add depth to the story as the heroic characters step in to protect or assist them.
She might imply that his stories are overly detailed. He could be including too many elements or going into too much depth, making the stories seem 'large' in terms of the amount of information.
Most literature focuses on more positive, relatable and acceptable themes. Stories about bullying are common, but this specific and rather vulgar - sounding combination is not something that pops up in traditional literary canons. However, in some alternative or shock - value - seeking independent writing, there might be something remotely similar, but it's not a widespread or well - documented area in literature.