The scope of a topic depends on how much time and space you have to write and how much detail you are trying to use. For example, describing all the causes of World War II in three pages is impossible. You would have to either narrow your topic some more or write hundreds of pages to adequately discuss every cause. Defining your topic before you start writing will save you time and help you to research and/or to develop your thinking in a clear, methodical way.
It is important to examine the topics we choose to determine whether they are too broad (or, in some instances, too narrow) for the writing assignments we are given. Once you have decided that a topic is too broad to be appropriate to your assignment (which is most often the case), you will need to have ways to narrow it. You will also want to consider, when writing essays that require research, how your research resources and limitations affect your choice of topics.