Context for this question is that we're thinking of using a hard-cover book as a guestbook for an upcoming wedding. We would be putting polaroid pictures inside this guestbook. The hard-cover book is not designed for this, but is a textbook that's relevant to the couple. The worry is that putting polaroids inside the book would prevent the book from closing. I am wondering if case-bound books' binds are perfectly sized to the pages they contain or if there is some wiggle room. Also would placing a weight on top of the book for a couple of days allow things to "squeeze" into place?
Yes, they are, with a bit of slack. If you stick too much in, the book will become misshapen and eventually the spine will be damaged.
I just happen to have a stack of lab books on my desk. These are case-bound, and have had printouts stuck in (office paper, so thinner than polaroids).
Even the bottom one on this stack is somewhat stressed. The middle one is probably twice as thick as a new book, and the top one has got so stressed from being overstuffed and used repeatedly that the spin has come off completely.
If you want to just stick a few photos in, you can get away with it, but if you're thinking of photos on a lot of the pages, a different solution would be better. I also have holiday note/scrapbooks at home. These were typically made using notebooks that were themselves a souvenir, and get quite misshapen. But they're better if you keep the photos away from the spine, and distribute them over the area of the page. The worst case would be a full-width photo at the top of every page, with writing underneath. Putting photos face-to-face isn't always a good idea anyway, certainly not with inkjet printed photos, so photos on facing pages should be offset in case they stick together.
If you're starting from a textbook rather than a book designed to be added to, apart from not having much space for writing, you may find the binding is tighter to start with, leaving less room for stretch