This is a marketing ploy to entice an author to use the print services. Obviously this method of design is not customized to a book's content. Mostly, it is a process of rehashing templates and changing colors. Design is especially critical when it comes to your book's cover. The following book sizes will cut your printing costs dramatically.
Printers buy paper in bulk at certain sizes.
Page size is contingent on the most economical fit for the paper that can also be accommodated on press. You can print a size different from those listed but it will cost extra for prepress and press adjustments. These sizes are categorized according to their most common markets. The negative, or film, of that signature is transferred to photosensitive paper, called bluelines, which are used to give you a dummy of your book for final approval. The cover is also color-separated and recomposited as a proof for your approval.