There are several explanations for how women achieved the wasp waist of the Victorian era. One thing is that, most women did not achieve the 18" waists we assume all women had, and the women who did were invariably very young and very wealthy. Often the small waist was actually a corseted 24" on a young woman. This seems less extreme when we see that today the uncorseted waist of a small woman can be between 24 and 28" based on standard size charts. I for example have a 27" waist, I'm 23, small, but of average weight. The women of the 18th and 19th century also didn't only wear corsets, they wore padding and undergarments such as bum rolls which padded out their breasts and hips and made their waists appear significantly smaller, when they may have not been so very different from many women's sizes today.
There is also the problem of what we view when we imagine the standard figure of 200 years ago. Often the figure we imagine comes from a drawing, and has no more to do with an actual woman's figure than today's photoshopped images.
Though many women faced malnutrition and even starvation during the 18th and 19th centuries, which did influence their size and shape, women who were of the poorer classes were also more likely to have many children in a short time period, and were less likely to be corset trained in adolescence which would have lead to a diversity of body shapes, just as we see today.
Many of the dresses which have been preserved in museums such as the Metropolitan Museum of Art and the Victoria and Albert, have a waist measure between 20-27" with the average being about 24". It is important to note that nearly all these dresses belonged to young, wealthy women. Women who were very unlikely to ever have experienced malnutrition. Rather, these women had such small waists because they were young, had not yet borne children, and many had been corset trained since at least early adolescence. Many of these dresses were not for every day wear. Even some middle class women would have worn and altered their clothing until it wasn't wearable, and likely would have reused the fabric as rags. Wealthy women's clothes and some made for middle and upper middle class women. Very few examples of working class or lower middle class women's clothes exist. Many of the dresses which have been studied were dresses which were worn likely only a few times, possibly because women grew out of them.
When we look at sewing manuals and pattern cutting manuals from the 19th and early 20th century, especially those aimed at older women (over age 25), or at middle and working class women, the diversity in sizing is greater than one would expect were all women incredibly small. For example, the attached "Weinberg's Cutting Manual", published originally in Yiddish in the Lower East Side of Manhattan, gives standard grown women's measurements up to a 37" waist, above the average waist size for American women even today. The clothing described in the manual were also quite fitted, and it would have been assumed the women would have been corseted, and likely also wearing dress padding and/or a bum pad. The patterns were not for loose house dresses for the poor housewife, they were for fitted suits for immigrant women entering the work force and the lower middle class. These women were more likely to have experienced malnutrition, but were also less likely to have been corset trained, and were more likely to be slightly older or have given birth.
Arnold, Janet. “Patterns of Fashion 2: Englishwomen’s Dresses & Their Construction C. 1860 - 1940”,1982.
Steele, Valerie. “The Corset: A Cultural History”, 2003.
Kim, Alexandra and Mida, Ingrid. “The Dress Detective”, 2015.
Bradfield, Nancy. “Costume in Detail: Women’s Dress 1730-1930”, 1968.
https://archive.org/details/weinbergscloaksk00wein/page/n4
https://www.metmuseum.org/about-the-met/curatorial-departments/the-costume-institute
There is always more to be said, but you may be interested in my past answer to It seems the most common waist size on Antiquedress.com is 22”. I’ve heard a few things about why this might be ... was it maybe actually common to have a 22” waist 100-200 years ago?