Is that really true that putting "your light source closer to your subject, makes the light much softer"? I thought it was the opposite.
A part of the answer also is as illustrated. The apparent size of the softbox is similar with respect to the model. A larger one further away has the same apparent size as a smaller one closer up.
A larger one closer is softer than a smaller one close up. Inverse square law applies. Light falls off, spill factors in.