It can be used as an insult (jokingly) when someone is behaving in an overly macho way.. person 1: "I can do 80 push-ups and not even get tired." person 2:: "Ok, big guy!" *laugh* It can be used to refer to someone who is tall or weighs a lot... "He's a big guy" It can also be used affectionately, girls might call their husbands "big guy" or a father might jokingly call his child "big guy". "Come on big guy, time for bed."