Skinny=beautiful is a recent western concept, most cultures still see fat as healthy, beautiful, lively, or desirable and skinny women as diseased, ugly, barren, and close to death. Beauty is in the eye of the beholder.
A good illustration of Fat as ugly or sexy is in the Soldier Son Trilogy by Robin Hobb.
I admit that a large part of this was a rant, I skimmed your Q and I may have been unfair and over-reacted. Also, I just saw that you are a recent member, in Writer’s we tend to be nicer than in other SE sites and less-judgmental, particularly for new users, so mea culpa.
After rereading I saw that you that you mentioned other cultures standards, and that you used “for the most part”.
(because based on American’s rap culture, strategically placed, fat seems highly desirable, and having lived in the US I knew many people who though of fashion models as walking skeletons and preferred more rotund people. I guess I just didn’t like my interpretation of those “our standards”, and that “Hey guys” was the drop that pushed my buttons the wrong way.
Anyways, here are some beautiful descriptions, some of them about unconventional beauty
“She possessed a beauty much different from Daisy, more like a wildflower in the unspoiled earth than a prize-winning rose in a formal garden.” ― Gary Inbinder, The Flower to the Painter
I also found a mention of bigger girl lit featuring fat protagonists like in in Good in Bed, Conversations with the Fat Girl, The Way It Is, and Alternative Beauty.
Since it is a foreign culture, a fantasy culture I assume, readers should not expect western cultural standards; so any descriptions you make should be taken at face value, they would be a natural part of that society and not "weird" in any way.
You can just use classical descriptions of beauty and substitute what is beautiful to that person and expend it with sexy fat terms. Something like: the heavenly softness of her ample thighs, the inviting curve of her round belly, the glow of her moon face, her lustrous thick arm, voluptuous curves, full-figured majesty, plump over-ripe breasts, motherly/womanly form, imposing body, big as a mountain; filled with honey-dew, shapely as a fathered pillow, chubby , curvaceous body, comfortable folds, full lips, prominent bottom, generous features, land of plenty, country of milk and honey, mountain of beauty, abundant gifts, liberal size, gorgeous ampleness, stunning presence, picturesque curvature, pleasing softness, superb figure, pleasing roundness, encompassing arms, spherical perfection, substantial form, profuse gifts, wide comforting chest, portly padding, stout flawlessness, striking splendor, grand fruitfulness, vast bosom, considerable attributes, bursting cleavage , shapely body, munificent shape.....
You could also build the differences progressively introducing the alternative standards of beauty and smoothly building up on it; this may be less jarring to readers unused to see things differently.
You wrote that you are afraid of loosing readers who could be weirded out. I think this relies more on the skill of the description that its subject. A good description can make literally anything seem beautiful. For instance, I used to dislike mushrooms, after reading Tolkien, I began craving to eat some raw, now I like them, and every time I eat one, I am transported back to middle-earth.
Jay, you argue
“But if the book is written in English, presumably the target audience is English-speaking countries, where for the most part the standard is "thin”
I am not sure about SE, but in similar sites user are up to 40% from India. The site is international and speaking English is no the same as sharing a mostly modern western cultural bias.
Here is the distribution based on an estimate on professional developers for SE. While modern Indians are influenced by western standards, traditionally Fat is a sign of health, welth and beauty. Read Indian women taking dangerous 'fat pills' to GAIN weight and emulate curvy Bollywood stars“
“Read “ it has never been considered attractive to be thin in India” , yes it is changing, but still, there are countries were girls try their damnest to be fat; read about forcefeeding or west Africa , or fat is beautiful, or Top 10 Countries Celebrating Female Obesity