DFRPG is rather wordy; the manuals themselves are flavorful enough to give a solid sense of the Dresden Files world and its playful-yet-serious attitudes. Sadly, this occasionally obscures the rules a little, but overall it's very well written.
Also, the book's got a lot of art.
General Rules with Examples (35+ pages)
FATE is a weird system for the vast majority of RPGers who are used to d20-style games, and DFRPG contains subsystems unique to itself even when compared to other FATE games. This means that nearly every section has at least one example of how the game looks at the table, and/or how a general rule might be applied in a specific instance.
Examples will pad most of the other categories here, too.
DFRPG isn't just about rules: it's a setting based on an established property. Volume Two is almost exclusively Creatures From The Books, and isn't necessary to play the game the way a D&D monster manual would be.
Extra DFRPG-specific Rules (80+ pages, conservatively)
DFRPG is crunchy (crunch = rules and mechanics; opposite of "fluff") for Fate games, and in the running for crunchiest. It splits many stunts off into multiple subgroups collectively called powers (see below for page count). Spellcasting is a relatively complicated system designed to be flexible yet balanced, and altogether occupies more than 80 pages in Volume One. Other subsystems, like the Hunger stress track for vampires and various transformative or magical options, are rolled into other pages counts (largely Powers and Spellcasting).
Campaign and Character Creation (125+ pages)
FATE doesn't just have the party make characters; campaign/setting/NPC design is a group task too. The process of city creation is about 25 pages itself; it's that important. Characters are self-designed rather than class-based, and the Dresdenverse runs the gamut from ordinary humans to werewolves, vampires, and holy knights. So the general rules for character creation and advancement take up 45+ pages (again, crunchier than normal for FATE), while specific options for templates, stunts, and powers are another 55+.
SevenSidedDie points out that just the powers alone "cover a tonne of supernatural abilities, some of them in several varieties, so that you can create new beasties and custom character types from scratch to suit your own Dresdenverse." That should give you a sense of the scope this book is trying to cover.
It's a GM manual too (50 pages)
Between Running the Game and Building Scenarios, they've got it covered for GMs.
Original Content: Baltimore (50 pages)
At least fifty pages of Volume One, and a good chunk of Volume Two, are dedicated to an original setting: a Dresdenversified Baltimore. It serves as an example and a campaign setting, and is very well designed.
Throw in a good sprinkling of art (including a full page for each chapter heading), a really good glossary and index, character/campaign sheets for both design and play, and you quickly get a doorstop for a manual.
The book's main bulk comes from examples and taking their sweet time to explain things. While it's good that they try to be so explicit, I think that often the book's style (while entertaining, readable, and good for getting into the mood of the game) gets in its own way when the rules should be presented clearly and concisely. A good solid editing with that in mind might have dropped the page count noticeably.
FATE is simple, but contains complexity
For comparison, the generic FATE Core book coming out next month will probably be 300+ pages. It's stripped of setting, but features sections on philosophy and how-to-make-your-own-setting content. Despite the mechanics of FATE being surprisingly light, teaching someone how to implement FATE without either losing him in vagueries or tying him to unnecessary rules is not a simple task--especially in a book rather than face-to-face.
The DFRPG manual tries to include setting design and game philosophy (for those who are unfamiliar with FATE from other sources) as well as dedicating a couple hundred of pages to its own intellectual property.