Where does this 1 bigger book is more profitable than 2 smaller ones comes from? If those two smaller books are more popular than the one then the two are clearly better.
We are talking about Tyranny of Dragons specifically. Delricho and I are suggesting that producing a single book for that AP would be more profitable than splitting the same content into two books and selling them separately. If you look at Elemental Evil, it follows that producing a single book (Princes of the Apocalypse) for this AP will be more profitable than splitting it into two books released a month or two apart.