A bigger source will cast a fuzzier, less distinct shadow. A single point source, infinitely small, will on the other hand cast a perfectly defined shadow with an abrupt edge. The shadow is likely to be a good image of the object that casts it. But imagine standing in the shadow of an object shading you from a larger light source, like the moon. As you move your head, sometimes the moon's disc will only partially be obscured, and sometimes completely. The area of the shadow where the light is only partly blocked is the penumbra. A limited region of the shadow is shaded entirely from the light source: this is the umbra. The specific effect of enlarging a light source is to make the penumbra bigger and the umbra smaller. More areas will receive less than 100% of the light, but a smaller area will be 100% shadow - hence the blurring effect. Light and shadow are more evenly distributed. Overall, the area in some degree of shadow will increase. Try observing the shadow of your finger under artificial lighting at home: the dark umbra should be visible, and the grey penumbra where the shadow fades continously away to nothing. The blurring will also be exacerbated by moving your finger away from the surface onto which its shadow falls. Note that it is the angular size of the source that really matters. The sun is huge, but since it is so distant its angular diameter is only half a degree, so it casts fairly sharp shadows.