I've recently read the Twilight series. They are all fairly big books, the smallest of which is still a good three hundred pages. Yet I look upon them now and see them undamaged. To the contrary, I'm also reading Mein Kampf, its around six hundred pages, but smaller than the largest of the Twilights. I'm currently at page two hundred, and the spine is cracked to oblivion. I eventually drew the graph below, using the formula of:
T x W = X
X= Spine cracking level
T= Book thickness (page count)
W= Word Heaviness
Basically, if the book is a decent number of pages (T) and it takes a while to read the words on the page (W) then the rate of spine cracking (X) will be small.