It's not size that matters. It's completeness.

Three examples:

A book contains a few sample chapters. Some readers will think of it as a bonus because it gives the reader a taste of the promoted book. Other readers are going to disagree since they must purchase the book in order to find out how the story ends.

Another book may contain a self contained story that is a preview of an upcoming book. It is still advertising material, but it does arrive at a conclusion. Many more readers will see this as a bonus since the reader is not obligated to buy something, even though it is intended to entice them to buy another book. (This is similar to a sample sized product mentioned above.)

The third example is a complete story, author interview, etc.. It's still there to sell a book, but is intended to sell the book that it is included with. This is a genuine bonus, in my opinion, because it is intended to create value for the reader. The other examples may add value for some readers, but their intent is to create value for the vendor/publisher/author.