The best I can guess is that they're typically smaller paperbacks sold cheaper (like around $6-8). I think the idea was to have a small travel size book. Many are romance books. But I'm not sure if there's any more to it because I never researched it much further--like, can you just sell a smaller version of your paperback copy?
Of course you could do a mass market edition and label it as such. You could do a special limited edition hardcover with gold embossing, too, the way some tiny presses in the sf/fantasy world do. But which company offers mass market size economically? Or gold embossing?
What we indies are mostly selling are trade paperbacks, which do not have a standard size because they don't have to fit into wire or other racks wherever they are sold. For most of us, their size is irrelevant because readers are buying them online anyway and have no idea what size the print version is. We choose a size we like, trying to avoid the look of a pamphlet if the text is short, and trying to avoid the look of a brick if the text is long.
