How do I get edges to form to the sprite shape and not a block around the sprite?

I am creating an angry Birds like game with Physics objects. I have set up the rigid bodies and box colliders around them. The problem I'm having is that when I install the physics objects i.e. Beams, Stones, columns, into the scene the edges around the objects are boxes. The edges don't fit tight around the sprites. As a result the objects push against each other when the game starts and the structures fall down before the game starts. How do I fix this?