As Matt said, much, much more.
I started from ground zero with the new book, spending the first four or five months simply reviewing and recording source materials before I wrote a single word. Over the course of a year and three quarters I worked my way through about 300 Traveller/GDW/Mongoose magazines, plus I have no idea how many interviews, podcasts, news reports, press releases, forum posts, and other sources. (They're all referenced on many, many pages.) Designers & Dragons was never used as a source, though I did check it here and there to make sure that my understanding was consistent and I wasn't missing anything. (But if I found something that needed more attention, I was out to other sources once more.)
Of everything, I feel like the DGP material might be the closest in content between the two volumes, but the word count is still likely 50% higher (it's a little hard to count because it's scattered amidst two chapters). Everything else is much more extensive. There's more on the origins of Traveller, there's more on its development at GDW, there's more on what caused the various crises in the line, there's much more on Imperium, there's material on fandom, on HIWG, and on a lot of smaller publishers during the "Long Night" that was never even touched on in Designers & Dragons, and there's more on Mongoose's two iterations of the game and some of their ups and downs (much of which postdated my finalization of Designers & Dragons over a decade ago). And, of course, it's all in cohesive singular narrative.
The new book came in at 112,000 words. In contrast the core histories in Designers & Dragons were 10,500 words for GDW, 3,500 for DGP, 4,000 for Imperium, and 8,500 for Mongoose (and of course the first and last of those contained discussions of lots more than Traveller).
I look forward to reviewers considering the book on its own merits, but personally, as an author, writer, and historian I think it's the best book I've ever produced. And I'm thrilled those stars aligned for a game that I have such a fondness for and history with (but that's probably why they did, it was a very fun project).