You can see that reflected in the author earnings reports.
http://authorearnings.com/wp-content...unit-sales.png
Just consider that the main reason the big five's share appears to be growing is because they are buying other publishers and their share. However, if you look at the pre-acquisition market shares there is a net decline in the BPH share.
Also, because Nielsen, AAP, and other outfits measure share in dollar terms and BPH prices have long been going up steadily. And not just on the ebook side.
If you look at their recent reports they have stopped reporting mass market paperback sales (after years of reporting constant declines) and are instead reporting "paperback" numbers with slight increases as they reduce the number of books released as mmpb and shift those titles to the more expensive trade paperback format, a form of stealth price hike practiced by tradpubs big and small.
The tradpub side of the business is undergoing consolidation and has been since Bertelsmann bought Doubleday. And it will continue consolidating as the multinationals try to buy market share to prop up their quarterly financial at all costs. The main cost being less unit sales from their side of the industry and more by the smaller, more agile publishers both Indie and tradpub.
There is a war going on in publishing but it's not tradpub vs Indie: it is multinationals versus everybody else, publishers, authors, and readers.