Books, like people, come in many shapes and sizes. The question is, why do books vary in size? A lot of it to down to what the book is about and the target audience.
Small books for small hands
Kids’ books vary the most in size. They can come in a huge size as big as the child reading it, or sizes for small hands. Younger kids usually enjoy smaller books so they can fit them into their book bags and they can hold them comfortably.
Keeping the price down on the book
Sizes can always make a big difference on the RRP of the book. For kids books for example, longer landscape sizes such as royal landscape are good for illustrations that are longer lengthwise, the same for square books too – a few of these sizes can keep the price down, which is especially helpful for new or unknown authors.
Longer books appear less bulky
A book that would usually be very thick in A5 would be a more reasonable thickness in a royal portrait size. A smaller book could make the book double the thickness as you can’t get as many words on a page.
A lot of it comes down to personal taste and keeping the audience and author in mind. If an author is new, usually it’s a good approach to have a cheaper book for example. Overall, size doesn’t matter a great deal, each of us have a preference!