6. The author. This kind of ties into #4 from yesterday's post, but it deserves its own treatment. If the author of the title in question is a celebrity—even D-list (or lower, if such a thing exists)—that will generally translate to a larger buy than would be the case if the author were not already famous. I mean, come on, do you think Jodie Sweetin could have even gotten a book deal at all had she never been on Full House?
7. House enthusiasm. This is sort of an extension of #2 from yesterday. Simply put, certain titles generate more excitement in-house than others, meaning they are higher priority and are more likely to receive advertising dollars, co-op, a stronger marketing push, and so on. Not all titles are created equal, and those that receive more support from editors, publishers, &c will more often than not see bigger buys than those that don't.
8. Comp titles. As I've said before, your book is going to be compared to similar titles that have already been published. If you've published a book in the same genre before, you'll likely be compared to yourself; otherwise, a book similar in genre, seasonality, and format will be chosen. While comparative titles are only one factor among many, they do have an effect on the size of the buy for your book.
9. Awards. If your book won a major award in hardcover, the buy for the trade paperback will be significantly higher than it otherwise would be. The same goes for any of your new hardcover or trade paper titles that go on-sale shortly after you win a major award for any of your previous books. (NB: by "major award" I mean National Book Critics Circle Award, National Book Award, Pulitzer Prize, Man Booker Prize, and so on.)
10. Rep and/or buyer preference. Maybe the buyer is a huge fan of this particular book and wants to put it in special promotion (e.g. Barnes & Noble's "Discover Great New Writers" program or Borders' "Original Voices"). Maybe the rep feels so strongly about the title that his/her enthusiasm convinces the buyer to take more stock. Either way, significant interest on either party's part can drive the buy up.