BDLD (Big Dog, Little Dog) is when two completely different types of personal demeanors (Personality's) hangout. Example: A Large, Quite Man, hangout with a Hyperactive, Skinny friend. A very Shy boyfriend, being with a very Outgoing girlfriend. Big Dog, Little Dog doesn't necessarily refer to size, just Personality.
The term comes from the idea of older cartoons where a Tiny, Hyperactive Mutt, would jump around a very Calm Pitbull-Type dog as they walked down the street. Hence Big Dog, Little Dog.
(Person 1) Man, I don't know if I can be with my girlfriend anymore. She wants to talk all of the freakin' time and I just want to sit and watch TV. (Person 2) Yeah, you make BDLD couple. But if I was you I would stay with her, she is hot! (Person 1) Your right but damn is she annoying!