Have McKinsey and its consulting rivals got too big?

The big firms just need to be big and known. There value delivered isn’t in the PowerPoint explaining that cutting costs and increasing revenue will increase profits. The value added is that companies buy a stamp from a famous firm when they deliver staff cuts, reorganizations or other controversial news. As such it doesn’t really hurt to go for scale. They’re a standards org for headcount reduction.

Why though? Why a company with a board, which is already independent, would need some consulting? Isn't the board to convey ideas to CEO?

The board is neither independent nor objective or often really in touch with the business. Therefore they need cover from a big name firm

It's always nice to point to a third party and keep blame from c-suite or board.