HMOs are cheaper than other forms of health insurance due to lower provider costs. At least that's what an analysis of a 2004 study comparing HMOs to other forms of insurance discussed by Jason Shafrin in a post on Healthcare Economist says.

The difference amounted to 9.3%, with no measurable difference in utilization rates or risk selection between HMOs and other plans.

So, as an industry, HMOs are not more efficient because they are better at managing care or selecting risk, they are cheaper because they pay providers less. I would note that the analysis is based on data from the nineties, so perhaps a more accurate statement is that in the past HMOs were more efficient.

I don't know if that's the case today.