The light grey area around a source indicates the region of space required by the solver to inject the fields. It is important not to place monitors inside this region because the fields there do not have physical meaning. This region is determined automatically, according to the mesh and the position of the source, and it always covers 2-3 cells. Therefore, the only way to control the size of this region, if necessary, is by changing the mesh step in the vicinity of the source, which can be done with a mesh override region. You can read more in this link.