One reason may be the increase of the oscillator strength due to quantum confinement effect.
Pathik may be right. Another possible explanation for indirect gap materials is that due to the spatial localisation of the exciton, its wavevector k becomes undetermined due to Heisenbergs uncertainty principle, permitting transitions with no, or a wider range, of phonons.
I would like to propose another simple answer. Quantum dots are zero-dimensional systems, thus the density of states (DOS) become delta-fuction like. If the energy of light which you consider is in the resonance of DOS, the extinction ratio might be larger than the bulk value. Note, however, if the energy of light is outside the resonance, it must be smaller, and the average value integrating over whole wavelengths must be the same with that of bulk - this is due to the strict sum rule.
@Takashi ..If average of extinction coefficient of QDs is same as bulk then from where the question of large extinction coefficient of QDs has arrived?
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