Reference to Donald J. Trump, whose last name is spelled with a big "t" and a small "p". Play on "big talk, small penis". Refers to a person who makes loud and annoying claims to make up for deep insecurities.
"You know that Donald J.? He's the real deal, tells it like it is, right?"
"Naw, he just talks big. Big T Small p, know what I mean?"