In my experience, they're not tiny. I'm a young male who owned and worked in my own bar in a poor area of the bible belt, so there's those caveats--but I averaged about 8% in tips over 2.5 years.
My niece, on the other hand, averages over 20%.
My service was in all likelihood better than hers--at the time, we had fewer customers and therefore I was literally able to do more. Further, I'm well-liked by my customer base, and by my own measure I was an excellent bartender.
Anyway, I had a very large amount of customers who did not tip. I'd say > 30%.