I hate to have to start this way, but I've got to burst your bubble a bit: "the middle size" in Georgian English generally refers to height, when used in descriptions of humans. A woman who is "above the common size" is taller than average.

Okay, but plump. Plump definitely refers to body type/weight, but it's not an issue of fetishism: the standard of beauty in the eighteenth and early nineteenth century was simply different than today. "Plump" was understood as a generally positive descriptor of full-bodied attractiveness.

Her stature was mean [i.e. short], her hair of a dark yellow, her face round and full, her eye grey; delicate harmony being betwixt each part's proportion, and each proportion's colour; her body plump, white, and smooth; her countenance chearful, and like to her condition.

(From The Analytical Review, Or History of Literature, Domestic and Foreign, 1788)

[Harriet Smith] was a very pretty girl, and her beauty happened to be of a sort which Emma particularly admired. She was short, plump, and fair, with a fine bloom, blue eyes, light hair, regular features, and a look of great sweetness, and, before the end of the evening, Emma was as much pleased with her manners as her person, and quite determined to continue the acquaintance.

(Emma, by Jane Austen, 1815)

So it's not what we would consider "fat fetishism", because it was seen as normative sexuality, not a kink or fetish. The listmaker was simply indicating that these women were attractive and looked comfortably well-fed, instead of appearing hungry and thin. If you look at flattering portraiture of women, like this portrait by Francis Cotes in 1768, you can see the ideal on display: she has full, rounded arms and hands, her collarbones and neck muscles are smoothed out to the point that you can't really see them, her chin has a bit of a pouch under it, and her cheeks are filled out under the cheekbones. All of these typically read as undesirable body fat today, but were signs of health and beauty in the eighteenth century.

But don't fall into the trap of thinking that standards of beauty are as simple as one figure being attractive and others being ugly. It's not eve like that today - sure, January Jones came pretty close to what the modern standard would consider ideal as Betty Draper, but everyone knows that Christina Hendricks's Joan Holloway was incredible, too, right?

In the very late eighteenth and early nineteenth century, there was this idea that a woman - a lady - with a properly refined nature would be susceptible to emotion and rather delicate and ethereal physically. In contrast with the sexually appealing, soft and plump armful, women who tried to fit this stereotype endeavored to slim down. The satirical New-York-based magazine Salmagundi described them exaggeratedly in 1824:

In vain did I look around me, on my first landing, for those divine forms of redundant proportions, which answer to the true standard of eastern beauty - not a single fat fair one could I behold among the multitudes that thronged the streets: the females that passed in review before me, tripping sportively along, resembled a procession of shadows returning to their graves at the crowing of the cock. ... Judge then of my surprise when I was assured, not long since, that this meagreness was considered the perfection of personal beauty, and that many a lady starved herself, with all the obstinate perseverance of a pious dervise, into a fine figure. "Nay more," said my informer, "they will often sacrifice their healths in this eager pursuit of skeleton beauty, and drink vinegar and eat pickles to keep themselves within the scanty outlines of the fashions." - Faugh! Allah preserve me from such beauties; who contaminate their pure blood with noxious recipes; who impiously sacrifice the best gifts of Heaven to a preposterous and mistaken vanity.

This trend tied in with the contemporaneous idealization of death by consumption, which I wrote about here - someone who suffered from tuberculosis would end up pale, feverish, and thin, and if someone wished to appear the sufferer of (what was sometimes understood as) a nervous complaint brought about by too much emotion or genius, they would want to be thin. Beauty and fashion are not just about sex appeal.