Skinny is often read as good today. We like skinny jeans, skinny models and, apparently, skinny health reform. It is likely that the Senate, which has just rejected repeal-and-replace and repeal-without-replace bills, will vote on a “skinny repeal” of the Affordable Care Act. What does this actually mean, and what would it produce?

The proposal most often labeled “skinny” would repeal the insurance mandate for individuals and larger employers under the banner of choice and freedom — both standard objectives of conservatives. It also would repeal taxes on medical-device manufacturers and, perhaps, also on insurers, with the goal of reducing the costs that must be reflected in premiums.

On the surface, both of those changes seem modest and reasonable. But I can assure Congress, as a former insurance company chief executive, that they would lead to a bloated upscale version of the Medicaid expansion so hated by conservatives. The result would be not only the loss of coverage for millions of people but also an even bigger bill for the government to pick up.

You have to look at the dynamics of the insurance market to understand this. To survive, an insurer has to predict the risk of costly claims and to obtain sufficient enrollment to balance customers who need a lot of health care with enrollees who have few or no claims. This works because while an insurer can’t know the timing or severity of illness for an individual, it can estimate the average claims of a group of individuals fairly well.