Wider fibre cores produce a longer path for any given photon to travel as they bounce back and forth along the length of the cable. Since attenuation is a function of distance travelled, a wider cable produces a greater attenuation as the incidence angle increases for any given photon.
Because individual photons travel different distances along the length of the core, they will arrive at the exit point at different times. The energy density of the original coherent light source is therefore spread over a greater arrival time and having travelled varying distances and hence different attenuations.
This cause both spatial dispersion as the photons exit the core at varying angles and also temporal dispersion caused by the different exit times.