I wrote an article in Spring 2015 about Amazon Publishing being the second biggest selling publisher on the Kindle Store, according to Russ Graninetti the Amazon Vice President in charge of Kindle matters. That means that the term Big Six, which was buried when the two biggest of the multinational-backed trade publishers, Penguin and Random House, merged together. Yet with the rise of Amazon Publishing in the digital sphere, the name Big Six should be resurrected. Ebook sales are about 25% of the revenue of the Big Five and the Kindle is responsible for a large percentage of those sales, so with Amazon Publishing part of a Big Two with Penguin Random House it makes sense to talk of a Big Six, despite Amazon Publishing not being based in New York and selling relatively few print books.
The original article was lost in a crash in the MMMporium web server. The above short paragraph reflects the basic premise of its content.