A common mantras is to use as large a diffuser as possible, and to place it as close to the model as
possible. The desired result is for the light “wrap around the model.” A diffuser is a translucent material placed in
front of a light to soften and reduce its intensity.)
With a small umbrella, the light coverage is narrow and the contrast is a bit harsh. You will see quick light falloff across the torso and the length of the body. With a medium umbrella, the light is a bit softer and coverage is wider. With a large umbrella, the transition from light to shadow is more gradual. This is evident in the floor shadows and on the body. This smooth transition between lit and unlit areas is why it is referred to as wrapping the subject in light.
The previous is an excerpt from the book, Lights, Camera… Nude!