As Candide opens, its main character is living a sheltered existence, unaware of the harshness of the larger world. (This is not to say that these larger themes of injustice and irrationality are not already present even in that first chapter. Consider the perception of Candide himself, deemed as possessing insufficient noble status, to see how the very idea of nobility has been warped to the point of absurdity.) However, after Candide's expulsion, he observes the full scope of injustice present in the world, along with the dramatic swings of fortune that can bring suffering at any time.
I would suggest, however, that the most important factor about Candide's suffering is that it is not depicted as extraordinary within the setting. Indeed, compare his experiences with that of Cunégonde herself or the old woman, to give just two examples. Consider also the number of people whose stories are not told, either because Candide does not personally interact with them or because they have died.
These themes are particularly important to keep in mind given that Voltaire was deeply convinced by the problem of evil (one of the traditional critiques against Christian teaching) and found the idea of a benevolent God irreconcilable with the scale of human suffering in the world. This same problem of evil serves as a critical theme running across Candide and is expressed in the book's very depiction of the world.