The first portion of the story is the eposition, the way that Achebe has chosen to share the history and culture of the Ibo people. As such, it requires the greatest time and attention as the story teller works to help the reader understand what it is that is going to fall apart. The distinct culture of the Ibo and how they related to the world and to each other is intricate and the first part of the story is designed to explore this.
The second and third parts are shorter as they show the disastrous influence of the white man and the way that the Ibo are destroyed and even play a role in destroying themselves. This in some ways mirrors the rapid deterioriation of a society that took five thousand years to define itself and was destroyed in less than a tenth of that.