: Thicker books require deeper gutters (wider margins) because the bulk of the pages prevents the book from opening flat.
In publishing and design, the is the crucial extra space added to the inside margin of a layout to account for binding. 📖 The "Gutter" in Text & Publishing GUTTERING
The gutter ensures that text doesn't disappear into the "fold" (the spine) of a bound document. : Thicker books require deeper gutters (wider margins)