Social Darwinism says that society is like an organism and that individuals within that society are like individual animals in a state of nature, that is, these individuals survive OR NOT depending on whether or not they are fit for the situation into which they are born. That is about as succinct a statement as you will find on this subject, and it is packed with associated meanings. First the idea that societies are organisms dates back at least to Hegel and his dialectic, which (as you will recall) tells us that human progress is the result of antagonistic forces (thesis and antithesis) struggling for survival and emerging as a new synthesis. If you read Hegal deeply you will discover that "antitheses" are not like anti-matter, something that will cause annihilation when one becomes involved with it. Anti-theses are "problems" and mental constructs, the emergence of which into one's life is accomplished by chance, design, devine intervention, and several other ultimate causes. Hegel is murky and so are his "syntheses" which are supposed to be new things or ideas in the world. Suffice it to say, though, that everything in Hegel remains, but gets reshuffled and mixed up together, much like organisms eating their way across a petri dish, a savannah, a city-scape, or an epoch.
Also the GOP idea of Social Darwinism then assumes that the chance of birth is one of the problems of the individual, not of the society. You can deconstruct Social Darwinism along this axis: is responsibility for chance happenings the dialectic problem of society or individuals. They put it on individuals and completely turn their societal back on the unfortunates that come to bat with two strikes on them. Liberals take precisely the opposite point of view.
So, after a lot of philosophizing and word-smithing you come to the idea that "starving the beast" so you can drown it in the bathtub is just a metaphor for drowning the unfortunate and misfortunate in society because they are just too much trouble ... and by the way ... helping them is a burden to folks who were to the manor born or scraped and grappled their way to wherever they think they are above the rest.
Krugman's essay leaves the idea that a socially intolerable situation is going to develop and that's what the GOP wants to happen ... their version of the general drowning event. But, think about it. If the people who are closest to the fan finally realize that their butts are the next to be sacrificed, will they protect the fan or their own butts, or will they simply turn off the fan. Will they see government as the problem or not? The fact is that no one really knows, except you and I. We know that the history of revolutions ends with individuals watching all remnants of civilization and security and the good things of life disappear, and so they institute strong government to make things better. This happens always. It means that the social and political revolution the GOP thinks it wants will (if history and human nature are any guide at all) turn out to be more government not less, achieved through bloodshed, needless destruction of the civil infrastructure, and awful excesses directed at minority groups, women, and children.