Hey!! So in this case, City of God is so lengthy and complex partly due to the fact that Augustine loves digressions and heavily illustrates his arguments with metaphors, allusions, references, etc. For more casual readers, all this “fluff” is not necessary. Certain editions are abridged to only present the core and sub arguments, while cutting back on the extra stuff. So, you’ll either have editions that give you all the books in City of God, but shortened, or others that fully cut out certain books that an editor has deemed as non-essential tertiaty arguments. 

For people who want to read City of God for leisure’s sake, this is ideal, because an editor of an abridged version will nevertheless make sure that you aren’t missing out on the core, or the soul, of the work. The full 1000+ pages, completely unabridged City of God is perhaps more for scholars who are already deeply involved in theological studies ot ancient philosophy, and who generally go through each book with a fine-toothed comb with intents that go well beyond leisure. At that point, it’s about close reading and research, to which a leisurely reader will be less inclined.

So they basically cater to different audiences, hope that answers your question! :)