mouth too big for his balls
Similar to "too big for their boots" and implying that the person in question constantly talks about doing things which they would never have the courage to do.
Alex: Man I hate that prick Jared, next time I see him i'll beat his dick in the dirt.
Jonny: I doubt that dude, he hits the gym a whole lot and all you do is sit around playing Warcraft all day!
Jordan: Forget it Jonny, Alex just has a mouth too big for his balls.
Jonny: I doubt that dude, he hits the gym a whole lot and all you do is sit around playing Warcraft all day!
Jordan: Forget it Jonny, Alex just has a mouth too big for his balls.