Why is the shadow bigger than the object?

The shadow is bigger than the object only when the object (o) is nearer to the source of light. Let us consider a point source (P) in which the light emanates from almost a single point and goes out radically in all directions. As light travels only in straight line paths if obstructed by an object it creates a shadow. If we consider the light source to be at infinite distant, the light rays reaching the object will be parallel to one another and cause a shadow with the same size as that of the object.

If the light source is nearer to the object, only the light rays emitted at an angle greater than the angle made by the line joining the point source and a point on the corner of the object would go unobstructed by the object. So the rays travelling within this angle gets obstructed and a bigger shadow is cast. As the distance between the object and the source decreases, the angle of the rays to go unobstructed also increase i.e. size of the shadow also increases.