Weight is not easily estimated from photographs or even clothing. I think it is important to remember that bone structure and body composition often have more of an effect on weight than mere height does.
I'll offer up my grandmother as an example. Her figure sounds very similar to those of the oldest Grand Duchesses. At the time of her marriage, my grandmother was 5'7" tall and weighed 107 pounds. I inherited her wedding dress and have measured it. The waistline is 21" and the bustline is 30". The photographs I have of my grandmother in her wedding dress confirm that the dress fit loosely, so her actual measurements must have been smaller by several inches. She was married in the 1940's and the dress was a size 10. A modern size 10 would measure (without ease, usually several inches) at least 36" in the bust and 28" in the waist. A modern size 0 would be considerably larger than my grandmother's size 10 wedding gown. So, in about sixty years it seems that women have become quite a bit fuller-figured without necessarily becoming taller. My grandmother was 5'7" after all.
My grandmother was considered thin in her time, but the fashion then was for plumper and shorter figures. When my grandmother aged, she put on weight, topping out around 126 pounds. At that weight, she was quite heavy for a woman of her bone structure, despite her height, and despite the fact that she was technically underweight. She often bought a size 14 or 16 to wear yet she was still technically "underweight" at a height of 5'7" and a weight of 126.
So, my rambling point is that a very fine-boned individual will likely not weigh much at all. I would guess that even the tallest Grand Duchesses were not much over 100 pounds, even if that sounds dreadfully thin to us. Photographs of my grandmother always showed her to have a lovely figure. She didn't look bony or skinny at 107 with a 20" or less waistline. She still had curves. One thing that was noticable though was how tiny her wrists and ankles were and how thin her arms and legs were.
Another interesting thing is that people who are considered "slender" today are somewhat different in structure than those who were considered "slender" a few generations ago. A friend of mine who has very tiny bones (her ankle is smaller than my wrist), is only 5'2", and considered very thin and petite was much, much too large to fit into the wedding dress I spoke of.
Several years ago Marilyn Monroe's personal effects were auctioned. Despite her reputation for plumpness (she has even been accused of obesity!), I remember reading in People (?) magazine that the auctioners had real difficulty in finding any mannequins small enough to display her dresses. Likewise, I saw a display at the Metropolitan Museum of Art in 2003 of society clothes from the 1920's and 30's. I don't think any of the dresses displayed would have allowed for a waistline of more than 24 inches. Most looked considerably smaller.
People today really are (in general) much more robust than a century ago, so comparing weights from individuals who may be of similar height and each considered thin in her day may be misleading if their bone density/ structure/muscle mass are different.
As another example, even back when I weighed only ninety pounds, I could not get my grandmother's wristwatch to fasten around my wrist. It was an inch too short, at least. So, it is no surprise that I grew up to weigh considerably more than she did, even while wearing a smaller clothing size. Our body composition was very different.
I hope I didn't come off as a know-it -all in my first post. I just find this topic interesting and wanted to offer my perspective.
ETA: Why can't I type wristwatch? Okay wrist watch. Will that do?
