It is a common error to conflate “monopoly” with “bigness.” The social inefficiency of monopoly comes through a restriction of output and a raising of price. With genuine monopoly power, say through government legal protection, prices continue to rise (even faster than general price inflation) and quality declines. Examples are government schools and healthcare.
Bigness, on the other hand, can be obtained by superior service to consumers, offering them lower prices than the competition or higher quality. Examples are Amazon.com and Apple, Inc.
Here is a chart of the percent of GDP spend on healthcare. Americans spent 6% of GDP on healthcare in 1965 and 18% in 2011.
Here is a chart of the percent of a typical budget spent on food. Americans spent 17% of their income on food in 1960 and 10% in 2013.
Here is Tom DiLorenzo on Monopoly and Competition: