Why only a narrow light beam gives a spectrum after passing through a prism, while only the edges of a wide beam turn out to be colored?

In a wide beam, the dispersion spectra of neighboring thin points are superimposed on each other, and the result is white light. At the edges, this overlap occurs, so the edges are colored. The narrow beam is split into thin colored beams due to the different refractive indices for each color. These colored beams are spatially separated.