John Goodman & Thomas Savings write:

The Physicians for a National Health Program, for example, claims that Medicare has lower administrative costs than private insurance and is able to use its monopsony (single-buyer) power to suppress provider fees. The group, which is resistant to managed care, favors “Medicare for all” and endorses a bill to do just that by John Conyers.

Paul Krugman, writing in The New York Times, also argues this way. He points to a chart (see Figure I) which seems to show that Medicare per capita spending is growing at a slower rate than private insurance. Krugman, along with others, touts the slower rate growth in the Canadian health care system (also called “Medicare”). In recent editorials, both Krugman and Robert Reich have joined the call for Medicare for everyone.

Are these unconventional critics right?

No.  Read the whole post: Is Medicare More Efficient than the Private Sector? | John Goodman’s Health Policy Blog | NCPA.org.