In "A Rose for Emily" how is her physical description related to the theme?

Miss Emily Grierson is not described in anything resembling a remotely flattering fashion. By the time the special deputation from the Board of Alderman come to call on her regarding her taxes in the town of Jefferson, the narrator says that she is

[...] a small, fat woman in black, with a thin gold chain descending to her waist and vanishing into her belt, leaning on an ebony cane with a tarnished gold head. Her skeleton was small and spare; perhaps that was why what would have been merely plumpness in another was obesity in her. She looked bloated, like a body long submerged in motionless water, and of that pallid hue. Her eyes, lost in the fatty ridges of her face, looked like two small pieces of coal pressed into a lump of dough as they moved from one face to another while the visitors stated their errand.

She is compared, via simile, to a bloated body that has been under water: pale and puffy. It is a gross and cringe-worthy description. The words used to describe her are overwhelmingly negative in connotation: words like bloated, pallid, fatty, lump, dough, and so forth. She is out of fashion, out of touch, and out of shape. Her prime is long gone, as is the era which she represents. Her Old Southern values and ideas are also out of fashion and out of touch, and they demonstrate how quickly attitudes change with the time and that if a person cannot keep up, they will be left out. Emily becomes something grotesque because she cannot let go of her antiquated and outdated values.

A prominent theme in Faulkner's story is that the South was in a period of social change, and citizens like Miss Emily Grierson were becoming displaced by modernity.
Emily Grierson is described as an old woman wearing nineteenth-century clothing: a black dress with what sounds like a watch on a gold chain at her belt. Her ebony cane is gold-tipped, and the gold is tarnished, suggestive of its age. Her hair is a "vigorous iron-gray," and she looks corpse-like, "bloated, like a body long submerged in motionless water, and of that pallid hue."
Emily Grierson appears unhealthy, well past her prime, and she is making no effort to keep up with the changing times, much like the values she embodies. She is a relic of the Old South, an anachronism, with her African American servant and firm belief that her father's gentleman's agreement with the town will be honored, though he is long dead. Her murder of Homer Barron, too, is born of an old chivalric code that leads her to believe that her reputation is more important than his life.