Why seperate pipes are used for storm water and for sewage?

During flood, the heavy water runoff is termed as storm water. Therefore it is clear that the flow of storm water is much heavier than the sewage water. Thus the pipe line of storm water should always be bigger and wider than to sewage disposal pipe to control the heavy flow of water during flood. Otherwise narrower pipes may break due to the pressure of storm water. That is why,  seperate pipes are used for storm water and for sewage.