In places where you can see the image of the monopole in the ground the pattern looks narrow because the antenna looks longer (monopole plus its reflection, or part of it). This region is larger when the ground is larger.
It is because the ground plane comes so close to bottom edge of the monopole antenna and the currents will be in-phase so that the beamwidth becomes narrow.
The currents in the ground plane are the image of the monopole antenna. If the ground plane is large it is a good image, if the ground plane is small it is a smaller and imperfect image. Currents close to the antenna are more important than current further out.