What's confusing to me is that even most of the the print-on-demand services I've seen for self-published writers don't go below trade paperback trim sizes (5x8 inches).

There's a huge amount of indie fiction that really wants to be in pocket-size mass-market print format (for those buyers who prefer paper to ereaders), for ergonomics and some of the pulp aesthetic, but it's forced to trade paperback trim.