I think that the quality slider of volumetric settings acts more or less, like soft shadows : With no soft shadows, the limits of shadow appears really straight, like the volumetric shadows in your first picture. With setting of soft shadows, the limits are blurry and, very high soft shadows can make them almost disapearing, like volumetric shadows in your second picture. The problem is that volumetric sunlight are hard to set correctly and faking it with spotlight, directional or pointlight usually saves a lot of wasting time and pain...