Big being used to suggest body type or physical features of the person.
Lou being short for their name of: Louis, Lewis, Louise, Louann, etc.
"I'm doing good, thanks. Kind of hungry."
ONLOOKER- 'THERES THAT OBESE GIRL AGAIN. BIG LOU, I WISH SHE WOULD HURRY UP AND KILL HERSELF'
(we love u)