Some girls throw that big dick line just to make you fell good. When you get the eyes rolling in the back of the head, then your doing something.
Or when she grands her teeth and say "You big dick bastard" or "big dick motherfucker" then your doing something.
I got a buddy who had his ego inflated cause his girl would tell him he always made her cum, he was big,etc. Then one day she was really pissed and told all his friends in front of him,how small he was and how sex sucked, and how she just pumped him up so he wouldn't be insucure. Man it was the funniest thing ever.