Why does the book look too thick or too thin?

The 3D book's thickness is calculated from your page count (and paper), so if it looks too fat or too skinny, the page count is almost certainly wrong. Set it to your final interior page count and the thickness corrects itself.

The reliable way to get the right number is to format your interior first — it reports the exact page count and writes it back to your book. See Why did my final page count change?