Let me start out by saying I'd love John's take on this - "A Ticket for a Seamstitch" is one of his favorite books, and it'd be great to see what he could add...
The first book in the Henry Wiggen series, "The Southpaw," checked in at a hefty 350 pages - and small print pages at that. The second book, "Bang the Drum Slowly," was about 100 pages shorter and the print size was larger, too. Believe it or not, "A Ticket for a Seamstitch," the third story in the series, is 100 pages shorter still. And the print size is huge.
I think these pretty dramatic size changes can be easily explained. I think Harris probably realized "The Southpaw" was a bit wordy and dragged on in the middle. Consequently, "Bang the Drum Slowly" seemed to be a much more appropriate length.
Mark Harris' lengthy preface in the beginning of "A Ticket for a Seamstitch" explains why the third book is even shorter still.
In the preface, Harris explains how "Ticket" came to be published as a book. You can tell he harbors a lot of bitterness, too; apparently, his original intention was to have this story published in Life magazine, not a full-length book (which explains its lack of girth).
But Life rejected the story, and selected another instead. Harris doesn't hide his true feelings about that story, or its author. He thinks the story's rubbish and the author is a hack.
So basically, he "settled" for having the story published in book form.
It's pretty interesting. I don't really know what to make of it.