The size of the eyes changes with the intensity of light, to explain it in a detailed way, pupils dilate in the dark and become smaller under the condition that the light is bright. Dilated eyes occur when you are tired or under high pressure, or you are in fear. In addition, dilated pupils can also result from a number of therapeutic medications or drugs, biological or chemical toxins and poisons as well as diseases involving the nervous system. Also, Dilated pupils may also be a sign of a serious condition such as brain aneurysm, brain tumor, cerebral edema, increased intracranial pressure, intracranial hemorrhage, skull fracture and stroke. You should go to hospital to have an overall examination if necessary.
As a matter of fact, it is quite normal for us ordinary people to have dilated eyes from time to time. Because when we meet some special situations such as panic, surprise, or afraid etc. our eyes tend to be dilated. It is resulted from the muscles around the pupils. But it is just ok, and you can take it easy.