The further away the light source the smaller it is relatively
If I stood slightly sideways on one foot in-front of a six foot by six foot softbox, my body is going to be fairly evenly illuminated top top to toe. If the light from one side casts a shadow, the light from the other fills it in
If I stood slightly sideways on one fifty feet away from a six foot by six foot softbox, to are going to see shadows on my face, because the softbox isn't evenly illuminating it, essentially, it has become a "hard light"
Consider the sun - it is massive, but it is a long way a way, and when you see the hard harsh shadows it creates, you can see why we call that hard lighting
If we moved a lot closer to the sun, say stood on the surface of the planet Mercury, the sun would be a softer light source (al be it very bright and hot)
lighting consistency, and shadow definition is simply down to geometry - you have
- size shape and texture of subject
- size of light source
- distance from light source to subject
- directionality (is the light source going to bounce about the room)
You also need to factor in the inverse square law
Draw it on a piece of paper in those terms, and soon you will figure what shadows (umbra, penumbra) and evenness of lighting are all about