Why is the first part of Things Fall Apart the lengthiest?

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.