It's more than one-stop-shop services (see the GP), and it is more efficient in one way: One stop shop, specialized legal skills, etc. are more efficient for certain tasks than the services small firms provide. Also, it can be more efficient in another way: If I said low-wage software developers were more cost-efficient, what would you say? The same goes for attorneys.

I find myself on the side of big firms here, but small ones also are more efficient in some ways. My only point is that, for some services, there is an economy of scale.