My Shih-Tzu is the same way, and BTW I have a Irish Wolfhound living next door to me. My Shih-Tzu makes that dog cower down and bow to her, I am not kidding.
I think it is the high pitched bark that is making the Irish Wolfhound cower with its paws covering its ears. ; ) Shih-Tzus definitely fall into the plucky category which makes them so pleasurable as a pet and companion. You are a lucky pet owner. I was a cat person until April 7th of this year. Hero is my first dog and I lucked out. : )