big balls

you know when someone says "That took a lot of balls" or "i wouldn't have the balls to do that."

does this have any truth to it?

do you think that guys with bigger testicles are, on average, bolder/more confident/outgoing/or take more risks?

No

It's just an expression, and I'm not even sure if the word "balls" is being used in that context--referring to testicles.