According to physics the answer may be light.
When the amount of light is less, the size of pupil expands so that more light enters to our eye and we can see the object. 
When the amount of light is more, the size of pupil decreases so that the amount of light enters less and so can see the object with bright light.
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yes dats y i mentioned physics..
The reason is same for both
Then wat were u thinking b4?
thereason for bio is probably the ans of the body i.e. the sympathetic and parasympathetic actions
Actually it is the light which influences the pupil.its size is cotrolled by the iris dim light---- size of pupil increases bright light---size of pupil decreases
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