" How sad (also called "Playing the World's Smallest Violin" or "The world's Smallest Violin Playing Hearts and Flowers") is made by rubbing the thumb and index finger together, to imitate the movement of the bow. It is used to show lack of sympathy for someone telling a sad story: equivalent to a sarcastic "break out the violins. " (wikipédia)
" Playing the world's violin, is an insult you say to someone when they get whiny and sad. When someone is complaining about anything e.g. "I'm so alone", "No one likes me!", "I cut my finger!!!". People respond with, "Ooh, I'm playing the world's smallest violin." This is referring to the sadness of a violin tune. And the smaller the violin, the sadder the tune. When you say "I'm playing the world's smallest violin." you are being sarcastic. The person is all sad and whiny, and you're saying (basically) "I don't care how sad you are, what you're saying is annoying! Grow up!" " (Yahoo)
I hope it helps