I had a best friend who is a little under 5'9" tall. He was a little over 7.5 inches long. Because of his size he looked like he had a big dick. He worked out a lot. We were workout buddies. His attitude was based solely on his looks, and it wasn't confidence, it was arrogance. Superiority. We are no longer friends at all.
There is a difference between arrogance and confidence. The "nothing to prove" confidence is NOT arrogance, as far as I can see it. It means self secure, and able to interact and relate to others and feel good about it, and at the same time they also feel good about themselves while they are with you. That's also called charisma. Attracted to someone for a reason you almost can't quite pinpoint.
Big cock attitude can be a lot of things, but if it's the "nothing to prove" variety, I'd say it's closer to being grounded, centered, knowing who you are, and opening yourself enough to let others in and help them feel safe and secure while they are with you.
Bragging, showing off, inuendo about size, etc, all fall in the category of insecurity and a need to be arrogant.
I wish I was grounded and centered enough to be more charismatic. Sometimes I fall into the braggin category. I have to catch myself. I'm working on it.