For most traditional and casebound books the covers are extended beyond the edges of the book block to provide extra protection for the edges. These projections are called the squares. The amount of overhang is dictated mainly by appearance ~ larger books having larger overhang. They are invariably equal on head ~ tail ~ and fore-edges. Occasionally the fore-edge square is slightly greater in width. I consider it part of the bookbinder's style to develop his own sizing policy ~ which may well vary according to the style of the book.
In the Yapp style of binding the hard cover-board squares are replaced by a limp leather overhang.