Answer
Small seeds need light to germinate since they will die before reaching the surface/light if they are planted too deeply. Large seeds have enough reserves to reach the surface even if they are not "seeing" light when they start to germinate.
Work Step by Step
The key here is to see the different between small and large seeds-- the amount of reserves which each has --and to extrapolate to the need for light/photosynthesis, making light a useful signal to decide when to germinate.
